The concept of time zones isn’t exactly a Unicode issue, but it does relate to issues of globalization.
This blog entry from 4 Guys from Rolla explains the advantages of storing times/dates in UTC format vs. local time. The first one mentioned is that if your servers switch time zones, your data will still be the same.
Quick UTC Primer
UTC time zones are defined in terms of the Greenwich Mean Time (GMT), from the 0° longitude line estabilshed at Great Britain’s Royal Observatory, Greenwich.
If you live in London, then you live in the UTC or GMT. If you live Paris, which is one time zone to the east, then you live in UTC +1 (or one hour later than London). If you live in Philadelphia (Eastern Time Zone), then you live in UTC -5 (i.e. five hours behind London).
The idea of using UTC is to flatten time zones and place everyone in the GMT (London) time zone, but then add information about how many hours to add or subtract in order to convert to local time. If you have operations in multiple time zones, looking at the UTC time can help you determine the sequence of events much better than local time alone.
About Daylight Savings
Interestingly, even though most countries (except Japan) implements daylight savings in the summer, UTC does not. Right now (Aug, 2007) London is UTC +1 (1 hour ahead), but in the fall it will return to UTC 0.
For the Eastern Time zone inhabitants, the summer time zone is UTC -4, and will return to UTC -5 in November.
That means right now, my EDT time of 2:35 PM converts to 6:35 PM UTC (or 18:35 UTC in military time).
NASA – http://science.nasa.gov/Realtime/Rocket_Sci/clocks/time-gmt.html
Federation of the Swiss Watch Industry – http://www.fhs.ch/en/worldclock.php