EST is ambiguous - it can be either UTC-5 or UTC+10. Yes Americans, Australians have clocks too!

Assuming that you mean the Yankee version, then EST is *always* 5 hours off from GMT. ALWAYS. If you're interested in daylight "saving" (which, of course, doesn't exist) then you are also interested in EDT and BST.

This all gets very complicated, so you should just use DateTime and use unambiguous timezones like Europe/London and America/New_York.