On Sunday, March 13, most of the USA, Canada, and Mexico's northern border cities will begin Daylight Saving Time, moving clocks forward one hour. The majority of Europe and the UK will not follow suit for another two weeks, moving forward on March 27.
This means that you have two weeks of trans-Atlantic time offsets different from your normal expectations. I do enough webinars in the UK that I have become used to automatically adding five hours from my US Eastern time to calculate what time it is in London. But for two weeks, I'll be wrong.
There is nothing universal or "standard" about local time standards. Any locality can declare the local time to be anything they want. Here in the USA, we have some states that choose not to make Daylight Saving Time adjustments, so the offsets between them and other states varies during the year. This month the southern Russian Astrakhan Oblast will change its local time zone on March 27. Russia seems to create time zone changes with astonishing frequency. In 2014, the entire country changed from using a summer time adjustment and there are at least 5 proposals up for voting on additional time zone changes within the country.
All of this serves to underline the critical importance of giving your webinar attendees clear and unambiguous information on when your webinar will start. Never assume they can calculate the offset from some arbitrary time zone you pick. Never use a shorthand such as "Central Time" or "Eastern Time" -- Those have local meanings in many different continents.
Never assume your attendees will know what a full, official time zone abbreviation means either. For instance, here are some common abbreviations with surprising ambiguity and significantly different local times:
- BST - British Summer Time, Bougainville Standard Time, Bangladesh Standard Time
- CDT - Central Daylight Time, Cuba Daylight Time
- CST - Central Standard Time, China Standard Time, Cuba Standard Time
I recommend stating your starting time using a major city as a reference: "10am Los Angeles / 1pm New York." Better yet, supply your registrants with a conversion link so they can see your starting time already converted to their location. There are several online utilities you can use, but my longtime favorite is www.timeanddate.com - This site really takes the business of time seriously. They follow the latest time zone changes and Daylight Saving Time inconsistencies for locations around the world.
Use their Event Time Announcer to create a page with your event date and time converted for easy lookup and publish a link to the conversion page on your invitation, on your registration page, and in your confirmation/reminder emails.
Another great tool on their site is the World Clock Meeting Planner, which lets you put in cities and see a cross-reference table of time equivalents that makes it easy to pick a convenient time for all participants.
The onus is on you as the event organizer or marketer to get people to your webinar at the right time. Don't play a guessing game and don't make assumptions.