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 Ross-Ziegler Blip was a stratigraphic event on Earth and a number of other colonized planets...Ross-Ziegler Blip
“The Military Calendar is the standard timekeeping calendar used by the UNSC and virtually all of...Military Calendar
“A Cycle is a Covenant unit of time. It is one artificial day on High Charity. A cycle contains...Cycle
“Military Standard Time (also called MST) is the official name of the UNSC's 2400 hour...Military Standard Time