MTC

MTC (Coordinated Mars Time) is a proposed Mars analog to Universal Time (UT1) on Earth.

A Martian year is 687 days. Though dates given in Doom (2016) seem to relate to a 365-day Earth calendar for whatever reasons, or at least there are no date higher than 351 given in any Codex references. While the dates mention MTC, it doesn't actually conform to the actual MTC format. MTC only relates to hours, minutes and seconds (not 'years' or 'months'): the specific year dates listed in the game are standard earth dates based on the Gregorian calendar (including references to Terran months: October and December).

Background
MTC is defined as the mean solar time at Mars's prime meridian. The prime meridian was first proposed by German astronomers and in 1830 as marked by the fork in the later named by Italian astronomer. This convention was readily adopted by the astronomical community, the result being that Mars had a universally accepted prime meridian half a century before the of 1880 established one for Earth. The definition of the Martian prime meridian has since been refined on the basis of spacecraft imagery as the center of the crater in Terra Meridiani. The name "MTC" is intended to parallel the Terran (UTC), but this is somewhat misleading: what distinguishes UTC from other forms of UT is its, but MTC does not use any such scheme. MTC is more closely analogous to.

Use of the term "MTC" as the name of a planetary standard time for Mars first appeared in the Mars24 sunclock coded by the NASA. It replaced Mars24's previous use of the term "Airy Mean Time" (AMT), which was a direct parallel of (GMT). In an astronomical context, "GMT" is a deprecated name for Universal Time, or sometimes more specifically for UT1.

AMT has not yet been employed in official mission timekeeping. This is partially attributable to uncertainty regarding the position of Airy-0 (relative to other longitudes), which meant that AMT couldn't be realized as accurately as local time at points being studied. At the start of the missions, the positional uncertainty of Airy-0 corresponded to roughly a 20-second uncertainty in realizing AMT.

Formulas to compute MSD and MTC
MSD can be computed from the Julian date referred to Terrestrial Time (TT), as[37]
 * MSD = (JDTT − 2405522.0028779) / 1.0274912517

Terrestrial time, however, is not as easily available as Coordinated Universal Time (UTC). TT can be computed from UTC by first adding the difference TAI−UTC, which is a positive integer number of seconds occasionally updated by the introduction of leap seconds (see current number of leap seconds), then adding the constant difference TT−TAI = 32.184 s. This leads to the following formula giving MSD from the UTC-referred Julian date:
 * MSD = (JDUTC + (TAI−UTC)/86400 − 2405522.0025054) / 1.0274912517

where the difference TAI−UTC is in seconds. JDUTC can in turn be computed from any epoch-based time stamp, by adding the Julian date of the epoch to the time stamp in days. For example, if t is a Unix timestamp in seconds, then
 * JDUTC = t / 86400 + 2440587.5

It follows, by a simple substitution:
 * MSD = (t + (TAI−UTC)) / 88775.244147 + 34127.2954262

MTC is simply the fractional part of MSD, in hours, minutes and seconds:6
 * MTC = (MSD mod 1) × 24 h

For example, at the time this page was last generated (13 Jan 2017, 10:29:32 UTC):
 * JDTT = 2457766.93798
 * MSD = 50847.08508
 * MTC = 02:02:31