> The geo lookup would be expensive computationally Not really? There's not that many timezones, globally, and I think their polygon data is in OSM / could be

> The geo lookup would be expensive computationally Not really? There's not that many timezones, globally, and I think their polygon data is in OSM / could be obtained from OSM. Simply brute-force colliding a single point with all of them I would expect to be basically "instantaneous", but there are a number of trivial optimizations, like bounding boxes, that you could do to speed it up. (Point-in-polygon is not terribly expensive; it's O(segments) in your polygon.) That said, there are problems with that, as you mention. I just don't think "computationally expensive" is one of them.