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The times of various events, = particularly astronomical and weather phenomena, are often given in "Univer= sal Time" (abbreviated UT) which is sometimes referred to, now colloquially= , as "Greenwich Mean Time" (abbreviated GMT). The two terms are often= used loosely to refer to time kept on the Greenwich meridian (longitude ze= ro), five hours ahead of Eastern Standard Time. Times given in UT are= almost always given in terms of a 24-hour clock. Thus, 14:42 (often = written simply 1442) is 2:42 p.m. Sometimes the character Z is append= ed to a time to indicate UT, as in 0935Z (Zulu). This is referred to = as Military Time. When a precision of one second or better is needed,= it is necessary to be more specific about the exact meaning of UT. F= or that purpose different designations of Universal Time have been adopted.= In astronomical and navigational usage, UT often refers to a specifi= c time called UT1, which is a measure of the rotation angle of the Earth as= observed astronomically. It is affected by small variations in the r= otation of the Earth, and can differ slightly from the civil time on the Gr= eenwich meridian. However, in the most common civil usage, UT refers = to a time scale called "Coordinated Universal Time" (abbreviated UTC), whic= h is the basis for the worldwide system of civil time. During the win= ter months, UTC is the civil time scale for the United Kingdom and Ireland.= It is also equivalent to th= e civil time for Iceland, Liberia, Morocco, Senegal, Ghana, Mali, Mauritani= a, and several other countries.&n= bsp;
One can think of UT1 as being a time = determined by the rotation of the Earth, over which we have no control, whe= reas UTC is a human invention. It is relatively easy to manufacture h= ighly precise clocks that keep UTC, while the only "clock" keeping UT1 prec= isely is the Earth itself. Nevertheless, it is desirable that our civ= il time scale not be very different from the Earth's time, so, by internati= onal agreement, UTC is not permitted to differ from UT1 by more than 0.9 se= cond. When it appears that the difference between the two kinds of ti= me may approach this limit, a one-second change called a "leap second" is i= ntroduced into UTC. This occurs on average about once every year to a= year and a half. Greenwich Mean Time is a widely used historical ter= m, however, due to ambiguity, its use is no longer recommended in technical= contexts.
UTC does not change with a change of = seasons; however, local time or civil time may change if a time zone jurisd= iction observes daylight saving time or summer time. For example, UTC= is 5 hours ahead of local time on the east coast of the United States duri= ng the winter but 4 hours ahead during the summer.
Not all time zones observe daylight s= aving.