UT1 is a measure of time based on the rotation angle of the Earth as observed astronomically. It is affected by small variations in the rotation of the Earth, and can differ slightly from civil time on the Greenwich meridian.

UTC is the time distributed by standard radio stations that broadcast time, such as WWV. It can also be obtained readily from Global Positioning System (GPS) satellites. The difference between UTC and UT1 is made available electronically and broadcast so navigators can obtain UT1. UTC is the basis for civil standard time in the U.S. and its territories.

One can think of UT1 as being time determined by the rotation of the Earth, over which we have no control, whereas UTC is a human invention. It is relatively easy to manufacture highly precise clocks that keep UTC, while the only "clock" keeping UT1 precisely is the Earth itself. It’s desirable that the civil time scale not be very different from the Earth's time, so, by international agreement, UTC is not permitted to differ from UT1 by more than 0.9 second. When it appears that the difference between the two kinds of time may approach this limit, a one-second change called a "leap second" is introduced into UTC. This occurs on average about once every year to a year-and-a-half.

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