Pick a time and a set of zones to see them side by side, with a bar showing which hours fall in working time everywhere. Daylight saving is handled from your browser's own time zone database, so the answer stays right across the weeks when regions switch on different dates.
Automatically, using your browser's IANA time zone database. This matters more than it sounds: the US and Europe switch on different dates, so for a couple of weeks each spring and autumn the gap between London and New York is four hours rather than five. Fixed offsets get this wrong every year.
UTC is the atomic time standard every zone is defined as an offset from. GMT is a time zone that happens to equal UTC in winter and shifts to UTC+1 during British Summer Time. For scheduling, say UTC and mean it.
Because offsets change twice a year but zone names do not. "14:00 Europe/Berlin" is unambiguous all year; "14:00 UTC+2" is only correct for part of it.