As people leave the Earth, we will no longer be bound to the
exact length of the Earth's day. It is an ideal time to convert
to the metric system instead of the hoursminutesseconds system
based on multiples of 12 and 60.
People on Mars could use decimal fractions of the Martian day:
centidays, millidays, microdays. To aid its acceptance, units
close to hours and seconds are also proposed. This "Martian
Metric Time" is intended as an alternate to defining new
hours, minutes and seconds stretched to fit the Martian day.
It is only for convenience in everyday activities on the surface
of Mars. Earth time units should continue to be used for scientific
measurements.
If a significant number of people live throughout the solar system
before Mars is populated, it would be better to use the second
as the basic unit for compatibility with current scientific practice.
For example, use kiloseconds instead of hours.
PROBLEM
The Martian day is 24 hours, 39 minutes, and 35.238 seconds long.
It is impossible to divide it into an even number of hours, or
any other units; and still have those units be the same as on
Earth. Most Martian residents will want to be awake during daylight
hours, and can easily adjust their circadian rhythm to be synchronized
with the Martian day. They will want simple ways to measure time,
such as being able to set their alarm clocks for the same time
every day. Whatever habits the early Martian residents adopt
will gain inertia and be hard to change later. Before choosing
a system (or falling into the habit of one), we should consider
its usefulness to Martian residents after several generations.
It may be best to use two time systems; use Earth time for interplanetary
navigation and astronomical observations; and a new system for
general use on Mars, such as scheduling daily activities. Using
two different systems would not actually be any additional burden
during communications between planets. Since the roundtrip delay
time for radio signals varies from 8 to 40 minutes, radio operators
would want several clocks on their wall: local time, the time
their voices will be heard, and the time when transmissions now
being received were sent, and perhaps more clocks if multiple
time zones on either planet are involved. Clocks showing Earth
time would use hours, minutes and seconds; clocks showing Martian
time could use whatever units are chosen.
Since the Martian day and year are not synchronized with the Earth,
the Martian calendar could also be different. This is not considered
here.
MODIFIED MARTIAN METRIC TIME
To applying the metric system to the Martian day we divide by
100, 1000, and 1,000,000 to create the units: "centidays",
"millidays", and "microdays".
However, the current system of hoursminutesseconds is ingrained
in our thoughts and language, any changes will meet much resistance.
Indeed, the French tried to divide the day into 10 hours when
the metric system was introduced, but it was not accepted. To
ease the transition, also divide the Martian day into 25 "Martian
hours". (It is closer to 25 hours than to 24.) The use
of hours is not incompatible with the metric system: 1 Martian
hour equals 4 centidays or 40 millidays. These Martian hours
are only 2% shorter than earth hours. A (Martian) centiday is
equally close to 15 minutes, which is a useful unit. 10 microdays
is just slightly faster than a second, actually very close to
a resting person's heartbeat. Therefore, I propose the following
nicknames for these units:
"Hora"  (from Greek for hour)
 = 4 centidays 
"Quarter"  (from quarter hour)
 = centiday 
"Mil"  (similar to minute)
 = milliday 
"Beat"  (from heart beat)
 = 10 microdays 
Visitors to Mars who are accustomed to hoursminutesseconds,
or anyone uncomfortable with the metric system, could think
of dividing the Mars day into 25 "hours" (hora), divide
each hour into 40 "minutes" (mil), and each minute into
100 "seconds" (beat).
Using units as close as possible to the Earth units would simplify
casual radio conversations between the planets, as well as easing
the culture shock of new arrivals. As examples: Researchers
on Earth may ask astronauts to continue an experiment "after
an hour for lunch". A student on Mars may watch a video
tape of a one hour lecture from an Earth university. A new immigrant
could understand the phase; "We'll be ready in a 'quarter'
(quarterhour)."
PROPOSED MARS METRIC TIME UNITS
Unit & "nickname" 
Abbr 
Equivalent Mars times 
Equivalent Earth times 
Typical Uses 
DAY (Martianday) 

1 day = 25 hora 1 day = 1000 millidays 
24 h 39 m 35.238 s 1.0275 Earth days 

MARTIAN HOUR "Hora" 
4cd 
25 hora = 1 day 1 hora = 4 centidays 1 hora = 40 millidays 
.9864 hours 59.184 minutes 
as an hour: a meal, class, appointment 
CENTIDAY "Quarter" 
cd 
100 centidays = 1 day 4 centidays = 1 hora 
14.796 minutes (.9864 of 15 mins) 
as in "quarter after the hour" 
MILLIDAY "Mil" 
Md 
1000 millidays = 1 day 40 millidays = 1 hora 10 millidays = 1 centiday 
1.4796 minutes 88.775 seconds (2 Md @ 3 minutes) 
"just a minute" 
BEAT 
10µd 
100 Beats = 1 milliday 
.88775 seconds 
as a second, = 1 heartbeat (at 68 / min) 
MICRODAY 
µd 
10^{6} microdays = 1 day 
.088775 seconds 
fast clicking 
Writing Specific Times of Day
A day would officially start 10 horas (hours) before noon, and
extend 15 horas after noon. This fixes noon at a convenient
time: 10:00 . Times between 1:00 PM and 9:00 PM on Earth correspond
to the Mars times 11:00 through 19:00, an easy adjustment for
travelers from Earth. The day officially ends at 25:00. This
is a couple of hours later than on Earth, so fewer people would
be awake long enough to use the words "today" and "tomorrow"
incorrectly after midnight.
The written form for a time on Mars would be:


horas : mils 

or: 
horas : mils . beats 

or: 
horas : mils . microdays 
The corresponding way to write an Earth time is:


hours : minutes : seconds . fraction 
Some common times of day are:
Martian Time 
Event 
(approximate Earth time) 
0:00 
official start of day 
(2:30 AM) 
4:00 
dawn 
(6 AM) 
5:00 
breakfast 
(7 AM) 
10:00 
noon 
(12 noon) 
16:00 
sunset 
(6 PM) 
17:00 
supper time 
(7 PM) 
22:00 
bed time 
(11 to 12 PM) 
25:00 
end of day, = 0:00 
(2:30 AM) 
Note that the first digit of millidays (mils) also represents
centidays or quarter horas:
Martian Time 
Name 
(corresponding Earth time) 
14:10 
quarter after 14 
(4:15 PM, quarter after 4 PM) 
14:20 
half past 14 
(4:30 PM, half past 4 PM) 
14:30 
quarter of 15 
(4:45 PM, quarter till 5 PM) 
For more precision, simply add on "beats" with a
decimal point; or add a decimal fraction of millidays using as
many digits of precision as needed. This is not ambiguous, because
beats are a decimal fraction of millidays:
Martian Time 
Name 
(corresponding Earth time) 
14:39.99 
1 beat before 15 
(4:59:59 PM, 1 second till 5 PM) 
15:00.01 
1 beat after 15 
(5:00:01 PM, 1 second after 5 PM) 
15:01.00 
one mil after 15 
(5:01:00 PM, 1 minute after 5 PM) 
15:00.003 
3 microdays after 15 
(5:00:00.3 PM, 0.3 sec after 5PM) 
Computer programs and digital clocks could use millidays for internal
computations. To convert a time to millidays, simply multiply
the number of horas by 40 (mils/hora) and add. For example:
20:07.50 = (20 * 40) + 07.50 = 807.50 millidays, or
0.8075 days.
Initially, everyone on Mars should use the local time (mean solar
time) of the first permanent base. As the population grows and
spreads out; they can decide if they wish to keep one universal,
planet wide time; or establish time zones.
Remarks
People often say they need more hours in the day, there would
be on Mars. The cliché "I'll get back to you
in a minute" might finally be accurate; no one on Earth
"gets back to you" in 60 seconds; so we can replace
the minute with the milliday, which is a minute and a half.
Many people have a natural circadian rhythm of approximately 25
hours; this gives me the eerie feeling that we were destined
to live on Mars instead of Earth.