Daylight savings time

Since the DST changes at different dates in europe and US, how do I easiest determine, based on a date submitted, what the current difference was?

If you need to know the offset for a timezone at a given date, I would suggest using the Date/Time node. You can pass in the given date, select the timezone, and then use the format operation with the format string ZZ (which will return you just the TZ offset).