Coordinated Universal Time (UTC) is the international time standard. It is the current term for what was commonly referred to as Greenwich Meridian Time (GMT). Zero (0) hours UTC is midnight in Greenwich England, which lies on the zero longitudinal meridian. Universal time is based on a 24 hour clock, therefore, afternoon hours such as 4 pm UTC are expressed as 16:00 UTC (sixteen hours, zero minutes).
Since a day is 24 hours long, the world may be split into 15 degree wide longitudinal bands (360 degrees/24 hours). Each band represents one hour. As an example, Huntsville Alabama is located at approximately 90 degrees west longitude, hence, local time lags UTC time by 6 hours (90/15, assuming Central Standard Time, 5 hours in Central Daylight Time). So, if the universal time is 14:30 UTC, United States Central Standard Time would be 8:30 am CST.
Illustrated below is a map which indicates the 15 degree wide longitude bands. Referring to the Huntsville example, count the lines west of the 0 meridian until you get near Alabama. There are 6 lines, hence 6 hours difference. Counting the bands may not give you an exact time for your location, as countries impose their own time regions due to geographic borders, but it should be close. Subtract time in west longitudes (negative values), add time in east longitudes (positive values).
Responsible Official: Dr. James L. Smoot (James.L.Smoot@nasa.gov)
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