Most professionals (astronomers, weather forecasters, pilots, etc) use a twenty four hour clock, referenced to Greenwich Mean Time (GMT), the time at Greenwich, England. The longitude there is 0 degrees; that's where the "Greenwich meridian" is. Greenwich Mean Time is also sometimes called Universal Time (UT). Note that GMT is always the same through the year (does not have a period of "summer" or "daylight savings" time).
To convert GMT to local time, or vice versa, we must know the time difference between GMT and local time. The time difference depends on the local longitude, and whether or not local time is standard or daylight savings time (one hour shifted). Not all states adopt daylight savings time in summer.
The Time Zones across North America and the Pacific from East to West are
Standard difference Daylight difference
Time from GMT Savings from GMT
Atlantic -4 hours
Eastern EST -5 hours EDT -4 hours
Central CST -6 hours CDT -5 hours
Mountain MST -7 hours MDT -6 hours
Pacific PST -8 hours PDT -7 hours
Alaska -9 hours
Hawai -10 hours
Here is a link that gives
longitude and latitude for major cities around the world.
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