From Halopedia, the Halo wiki
The Military Calendar is the standard timekeeping calendar used by the UNSC and virtually all of the human military. Based off of the Gregorian Calendar of the 16th century, the Military Calendar records the same twelve months and keeps the previous systems of counting years.
The Gregorian Calendar is based primarily off of the time it takes Earth to orbit Sol one time. This became a major problem once humans left Earth; every planet makes one full orbit at different speeds. Since a Year is defined as the time it takes the planet to orbit its star, timekeeping became non-uniform.
For example, it takes Earth approximately 365.25 days to orbit the sun, while it takes Mars 686.96 days to make the same orbit. The Military Calendar is intended to standardize the definition of a year and a month throughout the UNSC controlled space in keeping with the Sol definition.
The Military calendar answered the problem of a year, but the definition of a day faced the same problems; each planet rotates at different speeds, and on some planets, a day is longer than a year.
The Military Calendar uses two timekeeping systems to standardize the definition of a day, they are Military Standard Time and an anonymous "month-percentage" time system.