To ensure that the time in different regions is coordinated, a single standard is used worldwide. Universal Time Coordinated, or UTC, is a highly accurate time standard that is synchronized with the Earth's rotation and used globally to determine relative local times. It's also called Coordinated Universal Time.
What UTC Is
UTC is a time standard used worldwide. At any time, the current UTC time is the same as Greenwich Meridian Time, or GMT, the time zone used in England, western Europe and western Africa. UTC is used to calculate the times in individual time zones worldwide. To determine the local time of a time zone, hours are either added or subtracted from the UTC. For example, Pacific Standard Time subtracts eight hours from UTC. So if UTC is currently 11:25 a.m., PST is 3:25 a.m.
How UTC Is Determined
UTC is determined largely by the combined output of 200 atomic clocks. These clocks are located around the world, and their individual times are combined to determine International Atomic Time, or TAI. However, UTC also considers the minute differences between the accuracy of atomic clocks and how the length of a day varies based on the speed of the Earth's rotation. Because the Earth does not take precisely 24 hours to rotate, UTC periodically observes "leap seconds" every one to one and a half years. This additional second keeps atomic time precisely synchronized with the Earth's rotation.
Daylight Savings Time
UTC does not observe any seasonal changes in time due to daylight savings adjustments. Daylight savings time for individual regions is still calculated from UTC, however. A region that uses daylight savings time will add a different number of hours to UTC to determine local time while daylight savings is in effect. The daylight savings time standard for a region adds an additional hour to UTC. For example, Pacific Standard Time subtracts eight hours from UTC, so Pacific Daylight Time subtracts seven hours. During the summer, the United Kingdom does not use UTC as its standard time but rather British Summer Time, or BST, which adds one hour to UTC.
Zulu time is a time standard used by military organizations and in the navigation of planes and ships. Since military operations need to be coordinated precisely, different individuals need to be referencing the same time no matter where they are on the globe. The zulu time is always the same as the UTC.