Military Standard Time
Military Standard Time (also called MST) is the official name of the UNSC's 2400 hour timekeeping system. The Military Standard time system solves the problems of standardizing time throughout the UNSC space and is used in conjunction with the Military Calendar.
A day is defined as the time it takes for the Earth to rotate on its axis exactly one time. However, most planets do not spin at the same speeds, so once the UNSC began to colonize other worlds, the problem emerged that the definition of "years" and "days" became entirely dependent on the planet.
For example, one "day" on Earth is approximately 24 hours, but one "day" on Reach is 27 hours. The Military Calendar answers the problem of how to define a year, while Military Standard Time answers the problem of how to uniformly define a day.
Despite MST being the universal standard, some military forces stationed on Reach apparently followed the local 27-hour clock alongside the Military Calendar. The same may be true to other colonies as well.
There are apparently two ways the Military Standard Calendar is shown.
The most common is a standardized 2400 hour system. This system is used in most timekeeping objects, such as helmet recorders, UNSC watches and clocks. (It was also used at the beginning of each chapter of the five Halo books and all games). This system is based on the rotation of Earth; 2400 hours most likely corresponds to a standard Earth day (Where 0000 is midnight and 1200 is noon).
A second, more uncommon measure is only thought of as a "month percentage" time. It has never been explained and can only be inferred as recording the percentage of a month that has passed on the Military Calendar. An example of when this is used is on the UNSC Astrophysics papers, each is dated according to this system. The one made by SPARTAN-084 was published on May 097 and it can only be assumed that this refers to the end of may, after 97% of the month has passed according to the Gregorian Calendar used on Earth.