Coordinated Universal Time (UTC)
Most time zones are defined by their offset from UTC (which is essentially the same as GMT for civil purposes). For example, New York during standard time is UTC-5.
Daylight Saving Time (DST)
One of the biggest challenges in global scheduling is Daylight Saving Time. Because different countries start and stop DST on different dates (and some don't observe it at all), the absolute offset between two cities can change throughout the year.