http://www.worldtimebuddy.com/
This isn't entirely useful. For example, say an organiser says "The event will happen at 21:00 EST". Everyone then uses the above tool to convert EST to their local timezone.. and it still ends up being wrong.
Why?
1) They meant EST, but the program presumed EDT because DST is in effect (but not all EST locations observe EDT).
2) They actually meant EDT.
3) They meant EST.. Australia.
Far less mistakes happen when it goes:
Organiser: Known Local -> Common, Participant: Known Common -> Local
than when it goes:
Participant: Vague Local -> Local
And when organisers put
both the local time and the UTC time, it is easy to check if they got it wrong.