It sounds easy enough: build a clock that always shows the correct local time, no matter where you take it. Mobile phones can sort-of do this, but they manage by listening to the cell towers, so this won't work everywhere. "Radio controlled" clocks just listen to a low frequency broadcast, there's a few different transmissions across the world but they're incompatible and know nothing of time zones. Desktop computers can use the internet for time, but you still have to tell it what zone you're in.
Really, it should be possible to build a clock with nothing more than a GPS receiver, which figures out the time on its own, with no need for internet or cellular connection. It turns out that doing this requires an on-board world atlas, a spherical coordinate point-in-polygon algorithm, and a complete database of offsets and daylight savings rules.
I finally created a clock that automatically corrects itself as you carry it across country and timezone borders. The journey to this point was so ridiculously tedious that it borders on the absurd. In this talk I'll run through some of the highlights, and of course no talk about GPS time would be complete without mentioning leap seconds, which cause problems even when they don't happen.