Hi Andreas, I am working more or less on the same thing. What I ran into, is the following: the user´s local time settings are also unreliable. It happened to me. I have 2 clocks running, my first in European time and my second in South-American (where I am now) time. Most software checks the user´s “time”, thus time 1. That´s why all my appointments go wrong, they are 4 or 5 hours off.
It would be better to check the IP-block locale to guess what time it is in the user´s location. And always store as UTC and display behind it a local time based on IP time zone. International travellers would have less problems this way.