Universal Coordinated Time (UTC) is a time standard that uses the weighted average of hundreds of atomic clocks installed worldwide. Time zones are specified as UTC offsets, where GMT (Greenwich Mean Time in Britain) is UTC+0 and Toronto is UTC-4/-5 (for EDT/EST, respectively).
UTC is broadcast from radio stations, GPS satellites. Radio/satellite receivers can acquire UTC with an accuracy on the order of ~1 μs. These aren’t feasible in practice for distributed systems because receivers are expensive and don’t work indoors.