It's complicated. It's subjective (thanks relativity), really hard to write down (thanks timezones, geopolitics), inconsistent (thanks random jitter in the Earth's rotation), and worst of all, inevitable.
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.
Vernor Vinge, A Deepness in the Sky