Time for the annual warning to global webinar hosts, presenters, and attendees. We are about to enter the two-week period where the USA and Canada start Daylight Saving Time before the UK and Europe. That means for two weeks, your "rule of thumb" offsets are no longer valid: If you normally think of London as five hours ahead of New York, you'll be wrong for the next two weeks - it's only four hours ahead of New York until the UK starts DST on March 26. If you normally think of Berlin as 9 hours ahead of San Francisco, you'll be wrong for the next two weeks - it's only 8 hours ahead until March 26.
This is just another reason why you should never advertise your webinar or webcast time without adding a time zone conversion link. There are websites dedicated to keeping track of time zone offsets, including local differences in Daylight Saving Time (you will notice I didn't say anything about Mexico, which is too confusing to summarize). Don't force people to guess when another country's Daylight Saving Time goes into effect, especially when there are individual cities and states that choose their own offsets different from the rest of the country.
And fer goshsakes, if you advertise your times with a three-letter time zone abbreviation make sure you swap from ST to DT (Standard Time to Daylight Time) on the correct date!