If you've been running late recently, be consoled that the world, at least, has been running pretty much on time.

So punctual has the planet been, in fact, that astronomers at the U.S. Naval Observatory won't need to add their customary leap second on New Year's Eve to shim the earth's rotational vagaries into a proper astronomical day.

This is the longest such uncorrected period in 20 years and is weighty news in the world of inertial guidance and navigation, where time is distance. If your watch is fast, your missiles fall on the wrong people, among other difficulties.

To make sure that won't happen, observatory astronomers for the past 20 years have been, in effect, timing time. Using a battery of atomic clocks and four radio telescopes in Green Bank, W.Va., beamed on quasars, the most distant known phenomena in the universe, they confirmed that some days are, in fact, longer than others.

The Earth has slowed its spin rate so much over the millenia that it now provides us with days about four hours longer than those experienced by, say, a brontosaurus.

Days lengthen at the rate of something like one second every year to 18 months, but it's a wobbly sort of change, inconstant and only partly predictable.

To keep track of it, astronomers use a battery of atomic clocks -- suitcase-sized 170-pound instruments that look like something off the dashboard of the starship Enterprise.

While some 60 telescopes and observatories around the world and some two dozen satellites monitor the Earth's rotation to calibrate universal astronomical time, the atomic clock calculates atomic time, which ignores the Earth, sun and moon altogether.

The standard interval of atomic time is the International Second, defined in October 1967 by the 13th General Conference of Weights and Measures based on the resonant frequency of the cesium atom.

Atomic clocks operate by cooking atoms of cesium, a semiliquid metal like mercury, in a kind of miniature microwave oven. By using the number of microwaves necessary to boil off the greatest number of electrons from the cesium atom, and counting those electrons as they emerge, the atomic clock can tell when one atomic second has passed.

Atomic time is so uniform it varies less than one-billionth of an atomic second per day -- a standard of measurement among the most precise man has ever achieved. But it tends to get ahead of astronomical time as the Earth slows down.

Since the world's scientists have decided the two time systems should never get more than eight-tenths of an atomic second apart, they add a leap second to the atomic clock whenever sufficient divergence threatens.

The last leap second was added by closing out June 30, 1983, with a 6l-second atomic minute. At that point the world was running three-tenths of a second behind the atomic clock.

After the momentous insertion, the world was seven-tenths of a second ahead of schedule, but was expected to get back on time in succeeding months as its spin rate continued to flag.

Although that happened, it occurred slower than expected: so slow that nearly 18 months later the Earth is only one-tenth of a second behind atomic time.

Scientists prefer to add leap seconds to the atomic clock either at the end of the year or the half-year, but so far no such plan has been announced by the Bureau International de l'Heur in Paris, which arbitrates these things.

Astronomer Alice Babcock, who deals in Earth Orientation Parameters at the Naval Observatory, says the atomic clock will probably need a leap second June 30, 1985, but by then the world will have been on time longer -- 24 months -- than at any period in the past 20 years.

Why it should have slowed its slowdown isn't wholly understood, but then neither is the slowdown itself.

Part appears to be the mathematically predictable result of "tidal torque" -- the pull of the moon's gravity over millions of years.

Other variations, some fast, some slow, come from relatively predictable seasonal conditions -- the heating and cooling of land surfaces and the friction of winds on mountain ranges.

The global meteorological phenomenon called "El Nino," which resulted in a warming of the Pacific and massive consequent disruptions of the Earth's weather patterns, appears to have played a part in the current variation, Babcock said.

But that leaves a third set of variables over which astronomers and geophysicists puzzle: fluctuations in the rate of the Earth's spin that can't yet be accounted for or predicted.

Some hypothesize they come from volcanic upheavals within the Earth's molten core, shifts in mass that accelerate or retard the planet's rotation on its axis.

What patterns they will trigger in coming months, no one can fully say, but if present trends continue astronomers will have a new set of figures to work on -- the exact date some 5,256,000 years down the road when the Earth stops turning altogether.