Realistically, the first of our time systems that aliens are likely to encounter and associate with us is probably GPS time?

I sure as heck hope it is, at least...

GPS only goes up to 20 years though.

Ever heard of https://en.m.wikipedia.org/wiki/GPS_week_number_rollover ?

Unix time doesn’t include leap seconds.

As I said 6 months ago, "Naw that would make way too much sense": https://news.ycombinator.com/item?id=30855416