> Take the Traders' method of timekeeping. The frame corrections were incredibly complex - and down at the very bottom of it was a little program that ran a counter. Second by second, the Qeng Ho counted from the instant that a human had first set foot on Old Earth's moon. But if you looked at it still more closely ... the starting instant was actually about fifteen million seconds later, the 0-second of one of Humankind's first computer operating systems.
His descriptions of layers on layers of ancient code still running in that far future setting is somehow one of the most horrifying things I've read in scifi.
The Juche calendar counts from the birth of Kim Il-sung.
Unix counts from the Epoch.
The Islamic calendar counts from when the prophet Muhammad moved from Mecca to Medina.
The Ab urbe condita counts from the foundation of the City (i.e. Rome).
There are many, many more: https://en.wikipedia.org/wiki/Calendar_era
¹ Since Maimonides, before that it was from the destruction of the temple.
(Edit: Formatting).