## The Day

### Structure

How long is a day on Mars?

Just about any astronomy book will tell you that the rotational period of Mars is 24.6229 hours, or 24 hours, 37 minutes. However, note that the same table also gives Earth’s rotational period as 23 hours 56 minutes. But isn’t Earth’s day 24 hours long? What happened to the missing four minutes? The difference is that the 23 hours 56 minute figure is a sidereal day, i.e., Earth’s rotation is measured from the point of view of a fixed reference angle. But as Earth turns once on its axis, it also moves along its orbit around the sun, and the direction from the Earth to the sun changes slightly. It takes Earth an extra four minutes to rotate through this additional angle, and so Earth’s solar day, measured from the point of view of the sun, is 24 hours.

The same principle applies to Mars. Although the sidereal day is 24 hours, 37 minutes, the solar day is 24 hours, 39 minutes, 35.24409 seconds (88775.24409 seconds).

### 8 Primary Divisions

Several 8-hour systems have been proposed.

#### Con

The number 8 has few factors, being uniquely divisible by only 2 and 4.

### 10 Primary Divisions

Several 10-hour systems have been proposed.

#### Pro

Dividing the day by 10 is the first step of a decimal or "metric" clock.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

### 12 Primary Divisions

The only system in this category divides the day into successively higher powers of 12 up to 125. The system's basic unit equals 0.35677 standard seconds.

### 16 Primary Divisions

Several 16-hour systems have been proposed.

### 20 Primary Divisions

Several 20-hour systems have been proposed.

### 24 Primary Divisions

Several 24-hour systems have been proposed.

#### Pro

Retaining the 24-hour system that is used on Earth is the most conservative approach. The number 24 has many factors, being divisible by 2, 3, 4, 6, 8, and 12.

### 24 or 25 (Earth hour algorithm) Primary Divisions

The only system in this category adds an hour approximately twice every three days to account for the additional 39 minutes 35.2 seconds in a Martian day.

#### Pro

The system is based on the standard Earth second.

#### Con

Given the number of people who complain about switching between standard time and "daylight saving" or "summer" time twice a year, adding a hour twice every three days would be very disorienting.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

Source Documents:
 Martian CalendarAnton Sherwood

### 25 Primary Divisions

Several 25-hour systems have been proposed.

#### Con

The number 25 is a perfect square, thus it is only uniquely divisible by 5. There is no identified social use for dividing the day by 5.

### 30 Primary Divisions

The only system in this category is 74 minutes per hour, 40 seconds per minute.

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The system requires setting the clock back one hour every 180 days.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 32 Primary Divisions

The only system in this category is 40 minutes per hour, 40 seconds per minute. The system's basic unit equals 1.73384 standard seconds.

### 37 Primary Divisions

Several 37-hour systems have been proposed.

#### Con

The number 37 is prime.

### 60 Primary Divisions

Several 60-hour systems have been proposed.

#### Pro

The number 60 has many factors, being divisible by 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30.

### 100 Primary Divisions

The only system in this category is 100 units per day with no secondary units. However, chronometers would display to three decimal places. The system's basic unit equals 887.752 standard seconds.

#### Pro

This is a thoroughly decimal time system, being based only on powers of 10.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

Source Documents:

 A New Calendar and Clock for the Planet MarsManfred Krutein

### 360 Primary Divisions

The only system in this category is 60 minutes per hour, 60 seconds per minute. The system's basic unit equals 0.06850 standard seconds.

#### Pro

That is degree:minute:second, referring to angle between current zenith and Sun.

On another note, the division could as such be more calculable, using a different division of circle than our current 360:60:60 division. Hence, day division and year division could both be consistent with circle division.

This would mean that both day-division and year-division would constantly be moving, and the date would change simultaneously on the entire planet. However, the date would have a real meaning, as to location in the solar system.

We could be dark-based: 0 degrees of the year would be mid-winter, and 0 degrees of the day would be midnight.

Or we could be light-based: 0 degrees of the year would be mid-summer, and 0 degrees of the day would be noon.

### 1000 Primary Divisions

The only system in this category is 1000 minutes per hour. The system's basic unit equals 0.0887752 standard seconds.

#### Pro

• This is a thoroughly decimal time system, being based only on powers of 10.

• Avoids confusing differentiation between "Earth seconds/minutes/hours" and "Martian seconds/minutes/hours" by using a completely different nomenclature.

• Ought to be reasonably intuitive for metric system users, particularly scientists.

• Millisols are close enough (in length) to minutes to allow for fairly straightforward conversions when necessary.

#### Con

• The number 10 has few factors, being uniquely divisible by only 2 and 5.

• Confusing to new arrivals or visitors who are used to minutes/hours/seconds.

Source Documents:

 The Areosynchronous CalendarShaun Moss Martian Dreams Time SystemLinda Naughton and Robert O'Meara

### 8:8:8:8:8:8

The system's basic unit equals 0.33864 standard seconds.

#### Con

Source Documents:

 Martian Time KeepingDamon Dean

### 8:300:37

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The total number of seconds in this clock is 88,800, which is 24.8 more than a Martian day, thus the system requires "skip minutes" to be subtracted from the day on a regular basis.

The number 37 is prime.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 10:50:20

The system's basic unit equals 8.87752 standard seconds.

#### Con

Source Documents:

 Excerpts from The Gods of Mars and The Warlord of MarsEdgar Rice Burroughs

### 10:100:100

The system's basic unit equals 0.887752 standard seconds.

#### Pro

This is a thoroughly decimal time system, being based only on powers of 10.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

Source Documents:

 The Darian SystemThomas Gangale Le Calendrier MartienOlivier Prud'homme The Martian Calendar M. Vertregt Martian Year RevisitedM. Vertregt and Francis Graham

### 16:16:16:16

The system's basic unit equals 1.35460272 standard seconds.

#### Pro

Computers would be very happy... well, the AI ones at least.

Hex evaluation = 0xF : 0xF : 0xF : 0xF

One day = 0xFFFF (65536) Mecs or 2^16 power ( 2 bytes )

0xFFF  (4096)  Mytes
0xFF    (256)  Mims
0xF      (16)  Mours

16 Mecs in a Myte;  16 Mytes in a Mim;  16 Mims in a Mour;  16 Mours in a Sol
(21.67 Earth sec)   (5.78 Earth min.)   (92.47 Earth min.)  (1 Martian day)

### 16:150:37

The system's basic unit equals 1.00 standard seconds.

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The total number of seconds in this clock is 88,800, which is 24.8 more than a Martian day, thus the system requires "skip minutes" to be subtracted from the day on a regular basis.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 20:50:100

The system's basic unit equals 0.887752 standard seconds.

#### Pro

This is a modified "metric" clock. The basic unit is 1/100,000 of a day. The minute is 1/1000 of a day or "milliday." The division of the minute is based on a power of 10. The hour is provided as a familiar "human" unit of time.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

Source Documents:

 Martian Clock and CalendarMickey D. Schmidt

### 20:74:60

The system's basic unit is the standard second.

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The system requires setting the clock back one hour every 180 days.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

Source Documents:

 Martian Clock using Standard SecondsDale Shultz

### 24:10:370

The system's basic unit is the standard second.

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The total number of seconds in this clock is 88,800, which is 24.8 more than a Martian day, thus the system requires "skip minutes" to be subtracted from the day on a regular basis.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 24:10:1000

The system's basic unit equals 0.369897 standard seconds.

#### Pro

The divisions of the hour and minute are based on powers of 10.

The minutes on this clock are about 6.165 earth-minutes long. The 5-minute interval has proven to be such a comfortable length of time, that we rarely tell time in smaller intervals here on Earth, and we usually even take the extra effort to round to the nearest 5 or 15. When necessary, greater precision is accommodated by dividing the minute into 1000 seconds.

The format for this clock is hh:m:sss. "12:5" is clearly distinguishable from an Earth time, such as "12:30"). On Earth, 4:10 could mean either 4 hours 10 minutes, or 4 minutes 10 seconds. But with this Metric clock, you can clearly distinguish them by number of digits, since each field has a different number of digits.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

Source Documents:
 Martian Semi-Metric ClockAlan Hensel

### 24:60:43

The system's basic unit equals 1.433709 standard seconds.

#### Con

Source Documents:
 Time on MarsJohn J. G. Savard

### 24:60:57

The system's basic unit equals 1.081570 standard seconds.

#### Con

Source Documents:
 Time on MarsJohn J. G. Savard

### 24:60:60

The system's basic unit equals 1.027491 standard seconds. It retains the same units relationship as in an Earth clock.

#### Pro

The Martian day is only 2.7 percent longer than an Earth day, hardly enough for a person to notice. Similarly, stretching the familiar terrestrial units of time makes for a system to which people could most easily adapt.

The stretched 24-hour clock has been used by astronomers to describe the local time on Mars at least since Percival Lowell in 1894. The first functional Martian clock, developed by the astronomer I. M. Levitt in 1954, used this system. NASA/JPL adopted the stretched 24:60:60 clock for the 1976 Viking lander missions, and it has been used for all subsequent missions on the surface of Mars. This is one of the few generally agreed upon and commonly used aspects of Martian time today, and although not a de jure standard, represents a de facto standard established by custom and use.

#### Con

Source Documents:
 Time on MarsMichael D. Allison and Robert B. Schmunk The Darian SystemThomas Gangale Time Keeping on MarsAlan Dechert The Fortieth of JulyPierre Hallet An Easily Understood Calendar and Time System for MarsLeon G. Heron Rationale for the Martian Calendar's StructureBill Hollon MarsViewJoe Knapp Martian CalendarRoger E. Kuiper It's About TimeI. M. Levitt Mars Clock and CalendarI. M. Levitt Excerpts from Guide to MarsPatrick Moore The QS Calendar Jelmer Oosthoek Martian Clock and CalendarMickey D. Schmidt Welcome to MarsFrank Sorenson An Explanation of Zubrin's Mars CalendarPaul J. Thomas Options for Martian TimekeepingWilliam Woods A Calendar for MarsRobert Zubrin A Calendar for the Planet MarsRobert Zubrin

### 24:60:60 + 00:39:35.244

The system's basic unit is the standard second. A period of 39 minutes 35 seconds is appended to each 24-hour day.

#### Pro

This system retains the standard Earth second as the basic unit of time, and also retains the basic pattern of 24 hours per day and 60 minutes per hour. A second is the basis for measurement in so many scientific, technical, and medical procedures and equipment. We should preserve this link with Earth and its people. The people of Mars may decide later whether equipment, tables, and such should be modified.

#### Con

This system divides the Martian day unequally. The system would be difficult to coordinate among several distant locations on Mars, where the "time slip" would occur at different times. Since there is a fractional number of Earth seconds in a Martian day, the system requires "leap seconds" to be added to the day on a regular basis.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

It is unfortunate that most people seem to get their information about Mars from science fiction novels rather than from science books. The fact is that the stretched 24-hour clock has been used by astronomers to describe the local time on Mars at least since Percival Lowell in 1894. The first functional Martian clock, developed by the astronomer I. M. Levitt in 1954, used this system. NASA/JPL adopted the stretched 24:60:60 clock for the 1976 Viking lander missions, and it has been used for all subsequent missions on the surface of Mars. This is one of the few generally agreed upon and commonly used aspects of Martian time today, and although not a de jure standard, represents a de facto standard established by custom and use.

Source Documents:
 Excerpts from Red MarsKim Stanley Robinson Martian TimekeepingAnders Ström

### 24:(60:60 + 01:38 or 01:39)

The system's basic unit is the standard second. Each hour has 99 seconds, counted independently of the other minutes and seconds of the hour, appended to it after the last second of the last minute of the hour. This is to accommodate the longer Martian sol. Since the actual time needed to be appended is 98.9685 seconds, an excess of about 500 seconds will have to be eliminated by not appending all or some of the 99 seconds to a few hours each Martian year. The deducted amount is dependent upon the number of sols in the calendar year.

#### Pro

This system preserves a 24:60:60 clock using the standard second. For most civilian purposes scheduling to the second is not necessary and most Earth digital clocks do not display the second. Where accuracy to the second is required, Martian clock and watch displays can be enhanced to include a counter for the additional minute and 39 seconds added each hour. Standard Earth display of time in HH:MM:SS and use of Earth nomenclature for time can be used, providing one can live with ignoring the extra 99 seconds between hours.

#### Con

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 24:(60:60 + 99)

There is an "added seconds" counter, which counts 1 second (SI second!) every 36 seconds up to 100 per hour, while the ordinary seconds counter is stopped.

Each full 100 "added seconds" the "subtract counter" counts 1 second (SI seconds !), before the regular seconds counter continues. Balancing these counters gives the length of the Mhour 3699 seconds. In all calculations you need, mainly for navigation with speed and distance logging, you sum up the visible(!) number of seconds: hours times 3600 + minutes times 60 + seconds + "added counter" seconds - "subtract counter" seconds.

All clocks on Mars in each timezone get synchronized by radiosignal from "Airy beacon", giving MTC (Mars Time Consolidated) corresponding to UTC on Earth.

At each full Mhour there will be 1 second dropped or kept corresponding to the leapsecond needed, to nullify the 0.24... seconds per sol, which are not countable in units.

In case the clock misses some beacon signal there is no problem, because of the minor loss in accuracy per sol less than 1 second. The total sum of seconds counted is 88776, that is 0.76 seconds more than the correct number of 88775.24 seconds per sol.

Synchronizing the clocks on Mars with clocks on Earth is a special task for chronometer specialists, taking into account the different span of time needed for transmission at any time off any location! First we will need an atomic clock on Mars, counting in Milli Milli seconds! But watch out for Einstein's relativistic time shifting, stretching, squeezing in space travels with different speed.

#### Pro

This system preserves a 24:60:60 clock using the standard second. For most civilian purposes scheduling to the second is not necessary and most Earth digital clocks do not display the second. Where accuracy to the second is required, Martian clock and watch displays can be enhanced to include a counter for the additional minute and 39 seconds added each hour. Standard Earth display of time in HH:MM:SS and use of Earth nomenclature for time can be used, providing one can live with ignoring the extra 99 seconds between hours.

#### Con

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 24:60:(61 or 62)

The system's basic unit is the standard second. An algorithm is required to determine whether a given minute contains 61 or 62 seconds.

#### Pro

This system retains the standard Earth second as the basic unit of time, and also retains the basic pattern of 24 hours per day and 60 minutes per hour. A second is the basis for measurement in so many scientific, technical, and medical procedures and equipment. We should preserve this link with Earth and its people. The people of Mars may decide later whether equipment, tables, and such should be modified.

#### Con

This system requires a complex algorithm to determine which particular minute contains 61 or 62 seconds. The system would be difficult to coordinate among several distant locations on Mars, where the added second would occur at different times. Since there is a fractional number of Earth seconds in a Martian day, the system requires "leap seconds" to be added to the day on a regular basis.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

Source Documents:
 Excerpts from Martian RainbowRobert L. Forward Martian Clock and CalendarMickey D. Schmidt

### 24:60:61.64947506

The sol is divided into 24 major units (Martian equivalent of hours), which are in turn divided into 60 smaller units lasting 61.64947506 SI seconds (based on a value of 88775.24409 seconds for the sol). Expressed in Terran units, the "Martian hour" lasts 61.64947506 minutes, or 1 hour 1 min and 38.9685036 seconds. Clocks that display only Martian hours and minutes look exactly like Terran clock. Clocks that display seconds use the format 24:60:62 (i.e. display resets to zero at midnight after reaching 23:59:61).

#### Pro

Given that the second is almost never used in social and business schedules, and that many clocks do not display seconds anyway, the system will feel identical to the terrestrial clocks to most users.

No leap seconds or timeslips are needed. Time is counted continuously throughout the sol. Time-based calculations and unit conversions can be done essentially as on Earth, except that the factor 61.64947506 is used instead of 60 when converting between Martian “minutes” and seconds.

Keeping the SI second means that other units and constants derived from it (Hz, G, c, definition of the meter etc…) are the same on Mars and Earth. Physics textbooks and computer software don't need extensive rewriting on Mars.

The same method, i.e. stretching or shrinking the minute to express it in a decimal number of SI seconds) can be used to generate clock systems that use the second on other planets or satellites, not just Mars.

#### Con

Having to remember and use the number 61.64947506.

### 24:60:92

The system's basic unit equals 0.670103 standard seconds.

#### Con

Source Documents:
 Time on MarsJohn J. G. Savard

### 24:60:100

The system's basic unit equals 0.616494 standard seconds.

#### Con

Source Documents:
 Time on MarsJohn J. G. Savard Options for Martian TimekeepingWilliam Woods

### 24:100:100

The system's basic unit equals 0.369897 standard seconds.

#### Pro

The divisions of the hour and minute are based on powers of 10.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

Source Documents:
 Options for Martian TimekeepingWilliam Woods

### 25:40:88.7752409

The sol is divided into 25 major units (Martian equivalent of hours), which are in turn divided into 40 smaller units (millisols) lasting 88.77524409 SI seconds. Expressed in Terran units, the length of the "martian hour" is 59 minutes and 11.0097636 seconds. Clocks use the format 25:40:89 (i.e. display resets to zero at midnight after reaching 24:39:88). Noon (boundary between “am” and “pm”) is at 12:20:00.

#### Pro

No leap seconds or timeslips are needed. Time is counted continuously throughout the sol.

The major unit is almost the same length as the Terran hour.

This is a partially "metric" clock. The smaller unit (stretched minute) is 1/1000 of a day or "millisol" A quarter of the major unit is 1/100 of a day or "centisol."

Time-based calculations and unit conversions can be done essentially as on Earth, except that the factor 88.77524409 is used instead of 60 when converting between minutes and seconds.

Keeping the SI second means that other units and constants derived from it (Hz, G, c, definition of the meter etc..) are the same on Mars and Earth. Physics textbooks and computer software don't need extensive rewriting on Mars.

The same method, i.e. stretching or shrinking the minute to express it in a decimal number of SI seconds) can be used to generate clock systems that use the second on other planets or satellites, not just Mars.

#### Con

Having to remember and use the number 88.77524409 when converting millisols into seconds.

The number 25 can only be divided by 5.

### 25:40:100

The system's basic unit equals 0.887752 standard seconds.

#### Pro

This is a modified "metric" clock. The basic unit is 1/100,000 of a day. The minute is 1/1000 of a day or "milliday." The division of the minute is based on a power of 10. The hour is provided as a familiar "human" unit of time, while a quarter of an hour is 1/100 of a day or "centiday."

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5. The number 25, being a perfect square, has only one unique factor.

Source Documents:

 Metric Time for MarsBruce A. Mackenzie

### 25:50:71

The system's basic unit equals 1.000284 standard seconds.

#### Con

The number 71 is prime.

### 25:53:67

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The total number of seconds in this clock is 88,775, which is 0.2 less than a Martian day, thus the system requires "leap seconds" to be added to the day on a regular basis.

The numbers 53 and 67 are both prime.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

Source Documents:

 SCDTL Calendar and Clock for MarsLance Latham

### 25:60:60

The system's basic unit equals 0.986391 standard seconds.

### 25:100:100

The system's basic unit equals 0.355101 standard seconds.

### 37:8:300

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The total number of seconds in this clock is 88,800, which is 24.8 more than a Martian day, thus the system requires "skip minutes" to be subtracted from the day on a regular basis.

The number 37 is prime.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 37:10:240

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The total number of seconds in this clock is 88,800, which is 24.8 more than a Martian day, thus the system requires "skip minutes" to be subtracted from the day on a regular basis.

The number 37 is prime.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

### 37:40:60

#### Pro

This system retains the standard Earth second as the basic unit of time.

#### Con

The total number of seconds in this clock is 88,800, which is 24.8 more than a Martian day, thus the system requires "skip minutes" to be subtracted from the day on a regular basis.

The number 37 is prime.

The idea that "keeping the SI second is absolutely mandatory" is a common fallacy. In fact, scientists have never used the SI second to express the time of day on Mars. Rather, they have always used the stretched Martian second (equal to 1.02749125 SI seconds). People often fail to make the distinction between the use of the SI second for deriving other physical units, and the use of the second in relation to solar angle. Since even on Mars, a circle has 360 degrees, each degree has 60 minutes, and each minute has 60 seconds, it makes no sense to have a timekeeping system other than the traditional 24:60:60 clock. The relationship between time and solar angle must be preserved. Meanwhile, conversion between the stretched Martian second and the SI second is easily done whenever it is necessary. This is what scientists have always done with regard to Mars. Why do non-scientists insist that the SI second is absolutely mandatory when scientists know that it is not?

Source Documents:

 Martian Clock using Standard SecondsDale Shultz

### 60:60:60

The system's basic unit equals 0.410996 standard seconds.

#### Con

Source Documents:
 Options for Martian TimekeepingWilliam Woods

### 60:100:10

The system's basic unit equals 1.479587 standard seconds.

#### Pro

The divisions of the hour and minute are based on powers of 10.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

Source Documents:
 Options for Martian TimekeepingWilliam Woods

### 60:100:100

The system's basic unit equals 0.147959 standard seconds.

#### Pro

The divisions of the hour and minute are based on powers of 10.

#### Con

The number 10 has few factors, being uniquely divisible by only 2 and 5.

Source Documents:
 Options for Martian TimekeepingWilliam Woods

### 180° Longitude

#### Pro

This is consistent with usage on Earth, where the International Date Line is located at 180° Longitude, with adjustments for geopolitical boundaries.