Universal time is a world-wide standard of time observation which is based on converting local time anywhere in the world to the time along the prime meridian (0 degE, 0 degW) (see Latitude and Longitude). A knowledge of a universal system for measuring time allows observations to be made on a "synoptic" (from the Greek 'synoptikos' meaning 'general view of the whole') basis. This time reference is also known as Greenwich Mean Time (GMT) or Zulu time (Z).
Universal time calculation is based on the 24-hour clock (sometimes referred to as "military time" because of the military's exclusive use of the 24-hour clock). The 24-hour clock is used instead of the AM/PM convention and expresses hours and minutes in a four-digit group. Replacing the AM/PM designation digitally allows data to be recorded with time in a completely numeric fashion. This method can best be illustrated with examples.
Notice that any one-digit AM time reads the same except with a zero prefix added. Two digit AM times read the same on the 24-hour clock. All PM times are converted by adding 12 to the afternoon hour. The 24-hour clock starts at midnight with midnight being represented as 0000 hours.
The local 24-hour time convention is converted to UTC by adding or subtracting hours based on your location with respect to the prime meridian (your time zone) as well as local daylight savings time considerations. During "standard" time periods in the U.S., UTC can be found by adding 5 hours to Eastern Standard Time (EST), 6 hours to Central Standard Time (CST), 7 hours to Mountain Standard Time (MST) and 8 hours to Pacific Standard Time (PST). For example:
9:00pm EST = 2100 hours = 0200UTC (the next day)
When "daylight savings" time consideration is in effect (first Sunday in April through the last Sunday in October), the local 24-hour time convention is converted to UTC by adding 4 hours to Eastern Daylight Time, 5 hours to Central Daylight Time, 6 hours to Mountain Daylight Time and 7 hours to Pacific Daylight Time.
Next: Understanding UTC