Days, months and years... plus planets and seasons
If time were an accountant, its most treasured object would be a calendar. After all, keeping count of when things happen is one of the calendar's functions.
This is demonstrated by the very origins of the word, going back all the way to ancient times. The closest similar word is "calendarium", which means account book. Calendarium itself is derived from the Latin word calendae or kalendae, the first day of the month in Roman times.
Calendars have continually being altered and corrected and sometimes completely redefined.