In the novel "A Deepness in the Sky" by Vernor Vinge, the characters tell time this way. I had to do some math early in the book to get a feel for what 1Msec & 1Gsec was.

In addition, they also keep track of their cold sleep time, because if you want to meet your spouse 50 years from now, you want to try and keep the same relative age.

I was under the impression that the OS did only track time in seconds since the epoch, and whenever you see something else it's just the software translating for human convenience. If that's the case, then it this doesn't really make sense; humans will want something friendlier than epoch time on Mars, too.

Sort of. Unix time does not count leap seconds, all days are 86400 seconds long in Unix time. So the actual number of seconds since the epoch is a bit larger than what time() returns.

I don't know how Windows deals with leap seconds (its epoch is in 1601 or something like that). Edit: it doesn't, the clock goes 1 second ahead of UTC and it's then fixed by NTP.

So time() is actually days since epoch with 1/86400 precision. That makes the Google solution to smear leap seconds with NTP instead of making them explicit make even more sense.

Since the Unix Epoch where? Isn't being on another planet going to have to deal with the relativistic passage of time?

France used the decimal time for a few years beginning in 1792 during the French Revolution. It didn't go well.