Can I convert time zone calculations (18:30 CEST to GMT+1 for example) with Google?
You can do that with WolframAlpha.
EveryTimeZone is a nice webapp that gives you a really quick simple view of the time (you can drag the 'current time' around) in various common time zones
Google currently does not do time conversions but you can always ask Google the current time at a location. Just search for "time in
<city>" (e.g. time in london).
That is the way I use to quickly get the time in a different location. Of course you still have to calculate the difference yourself.
EDIT: I just found out that this trick only works if the "interface language" for Google is set to English.
Seems to be working now (in 2018):
I really like timeanddate.com's interface and the Time Zone Converter near the bottom works well.