Probably best to just drop the notion of "days", "years", etc. entirely, and instead just track the number of kiloseconds, megaseconds, etc. since the Unix Epoch.
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 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.