That's easy. We'd just use Greg time.

Greg's age and mood is highly deterministic, and he has atoms in his body present from the big bang. His sense of time varies, and seems to accelerate as he gets older, and he will tell you about it with extreme detail down to either 2 decimal places or 3 beers. If you call him up and ask him what time it is, the degree of the obscenities used in his reply is usually a good enough correction coefficient when calling over long distances.

Also two of his kids hate him, and his current wife is thinking of leaving him; all countable metrics that one can use to ascertain what stage in his life Greg is at, and thus what the local date/time in your area is, based on all the above Greg stats.

Don't we technically already have to account for time dilation for things like GPS satellites

Relief point: we will still be using unix timestamp.

Anxious point: we will still be using unix timestamp.

We're all just going to use tz_database and turn the maintainer into a ghost trapped in the machine for all of eternity.

tine dilation

Holds up fork and squints at it

I will just rely on ISO 8602 to introduce a universal time format

days may be longer than years (hello Venus).

Hello back

Nah, all this is solved with star-dates. One of the Federation's crowning achievements - Warp Drive is small potatoes compared to getting hundreds of delegates from as many different worlds to all agree on one calendar system that is not based on their own world's orbit around its primary.. Diplomatic impossibility, but they managed it.

Also, interplanetary timezones and leaps.