What about storing current geo coordinates and a timestamp of when they were recorded? I feel like that should be enough for a theoretical future system to adjust mapping for continental drift.

The problem with geo coordinates is reverse mapping them back into political owners, so you can then determine current timezone policy.

There's 4 things at play here:

   - Coordinate location on the planet
   - Identifier for that location
   - Political entity for that location
   - Current timezone policy

Of those, coordinates are the only thing that doesn't change over time (for the spans we're talking about).

But are also a pain in the ass to use. F.ex. I'm sure someone on HN has the lat long for London memorized, but I don't.

London is perhaps the only major city where you could be expected to know one of these coordinates. It's defined to have longitude 0.

I suggest giving date and time in the form of phase/frequency references to nearby pulsars. That should guarantee stability for a few million years.

Especially assuming an increase in computing intelligence in that time.

Europe as a whole is too. IIRC there's a set of points around the periphery of the continent that are measured to centimeter accuracy.