At 7 p.m. tomorrow, the U.S. Naval Observatory will electronically delay its clocks for one second. This means we will have an extra second injected into our lives.

One whole second of previously unscheduled time.

"There are two kinds of time scales we're using here," explains Dennis McCarthy, chief of the Earth orientation parameters division at the U.S. Naval Observatory (whose turn it is this year to explain concepts of time to members of the media).

"There's the time measured by the rotation of the Earth, based on the fact that one day is one rotation of the Earth . . . It is tied in with the definitions of inertial coordinate systems, adopted positions of the stars and quasars. The other time is atomic time."

Of course.

Atomic time, to modestly rephrase Mr. McCarthy, is based on the oscillations of a selected electron within a given cesium atom. Atomic clocks (of which there are 30 at the observatory) take cesium, a semiliquid element, and cook it microwave-style to get its electrons resonating. When the electrons shake themselves from left to right 9 billion times per second, it gives scientists a near-absolute way to measure time. Atomic time pays no attention to the Earth and quasars.

"The Earth has so many things affecting it that are unpredictable," says McCarthy, who has armed himself with a zen patience for the ignorant. "There are winds, Earth tides, the weather, and things we don't understand that are going on inside the Earth."

And therein lies the difference. Our Earth time keeps straying one way and then the other, shifting away from atomic time, at an average of one second every two years. The U.S. Naval Observatory and other time-keeping watchdogs around the world from Greenwich, England to Xian, China keep tabs on this dynamic inter-time fissure and make recommendations to the Bureau International de l'Heure in Paris. If there is a difference of more than nine-tenths of a second in a year, the French officials throw in un petit overtime to achieve UTC, or coordinated universal time (which takes at least a second to say).

The last time-adjustment was made June 30, 1983, when the Earth was circling a second behind atomic time, like a cosmic sluggard. In spite of our slow performance that year, however, the Redskins still managed to beat Miami in the Super Bowl.

Last year may not have been a great year for Democrats, but both atomic and Earth-rotational time were in relative sync. Athletes at the Los Angeles Olympics had no help or hindrance in setting track records. It was a good year for athletics.

But this year Earth has dragged its terrestrial feet again. Atomic time and Earth-rotational time have, again, parted ways. Hence the needed extra second.

It couldn't have come at a better time. Now, we can watch another second of the David Letterman show or read another line of Anna Karenina.

Forty winks can be pushed to 4l. Especially in the White House. We can dawdle in a Scrabble game and not be penalized.

That second, added earlier, might have been be the difference between a victory and a loss in the Celtics-Lakers battles ("And Larry Bird has saved the Celtics from disaster with a last-second dunk . . .").

Now TV networks can squeeze in yet another Bounty commercial.

Auctioneers can briefly be understood.

New Yorkers can briefly be understood.

Deejays can take one more call.

Defense contracting agencies can charge more time.

You can refer to Thomas P. (Tip) O'Neill Jr. by his full name.

Mick Jagger can finally be grammatical and sing, "I can't get any satisfaction."

Bruce Springsteen can sing, "Born in the United States of America."

You get the idea. But whatever you do with your extra second, remember Shakespeare in "Macbeth": "Let every man be master of his own time."