Time can only be measured by clocks (oscillatory systems). It's a slippery concept for which humans have a natural, flawed intuition. But if you understand your clock(s) really well, then timekeeping makes perfect sense. There's just a lot of clocks around. There's Unix time, UTC, TAI, smeared UTC, stop-the-clock UTC, UT1, and lots of others. Make sure you understand the clocks your system is using and how they relate to one another, and you can keep your sanity.
