Both the UNSC and the Covenant use separate, though analogous, measurements to regulate and measure the passage of time and standardise the calendars of their interstellar territories.
“The Military Calendar is the standard timekeeping calendar used by the UNSC and virtually all of...Military Calendar
“The Ross-Ziegler Blip was a stratigraphic event on Earth and a number of other colonized planets...Ross-Ziegler Blip
“Military Standard Time (also called MST) is the official name of the UNSC's 2400 hour...Military Standard Time
“The Covenant battle calendar is the standardized timekeeping calendar employed by the Covenant...Covenant battle calendar