There are three basic methods of keeping time in common use today. They include time based on 12-hour intervals (a.m. and p.m.); a 24-hour clock; and Coordinated Universal Time clock (called UTC or Zulu time). All of these have to do with simple mathematics—mainly addition and subtraction.

A 12-hour clock (using the 12-hour interval concept) is familiar to most of us. It represents half a day (24 hours divided by 2). The abbreviations “a.m.” and “p.m.”— terms that originated from the use of longitudinal meridians—are used to differentiate between the morning and afternoon hours. The term meridian (from the Latin meri, a variation of medius, or “middle,” and diem, or “day”) once meant noon. Thus, the time before noon was called “ante meridiem” and after noon was called “post meridiem.” They were eventually shortened to “a.m.” and “p.m.,” respectively. Whether the terms are capitalized or not is not an issue either, as both are used in texts. The 24-hour clock is, naturally, divided into 24 hour increments. It is commonly used by the U.S. military and other government agencies throughout many countries.

UTC (or Coordinated Universal Time; the letters are not a true abbreviation, but a variant of Universal Time) is equivalent to mean solar time at the prime meridian (0° longitude), formerly expressed as GMT, or Greenwich Mean Time. (The change was done to eliminate using the name of a specific location in an international standard.) Other names for UTC are World time, Zulu time, and Z time. UTC should not be confused with UT, or Universal Time, the basis for the coordinated broadcast of time signals, counted from 0000, or midnight.

A brass strip marks 0° longitude (the prime meridian) in Greenwich, England.