On the east coast, we're in Eastern Standard time or Eastern Daylight time. That puts us in either "zone +5" or "zone +4", meaning that to convert our time to GMT (as us old navigators prefer to call it) you have to add either five or four hours. For example, when we are on standard time noon here is 5 PM GMT, and during daylight savings time noon here is 4 PM GMT.
But even old navigators like me get rusty at doing this stuff in our heads, so to make life simpler we show the current GMT (or Universal Time) on the weather page. Satellite photos are usually tagged with the universal time of when they were taken, so if the satellite photo says "14:15z" on it, and our weather page says that the universal time is now 15:30z, then that tells you that the satellite photo is an hour and a quarter old.
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