The U.S. Naval Observatory plans to begin offering at 5 p.m. today a national telephone service that, for 50 cents a call, provides the time of day with millisecond precision.

The service would reduce the cost of obtaining the precise time for many callers outside the Washington area, while possibly increasing it for area residents.

Before today, Washington-area callers seeking the time of day could make free calls to the Naval Observatory's "master clock" telephone number, 653-1800, and the C&P Telephone Co. service, 844-2525.

On Jan. 1, C&P will end its free time and weather service as part of the federal antitrust judgment splitting local operating telephone companies from the national Bell system.

The Naval Observatory, which had been using 10 local telephone lines for callers seeking the master-clock time, said it will now immediately stop using five of the lines and perhaps cut back again later.

The observatory said it receives between 5,000 and 6,000 calls most days and as many as 20,000 calls on peak days, such as those on which most of the nation switches to or from daylight-saving time. One of those occurs Sunday at 2 a.m.

An observatory cutback in lines means that local callers who dial the master-clock number may receive a busy signal and may have to resort to the new 50-cent call to 900-410-TIME.

For callers outside the Washington area who want the observatory's highly precise time message, the 50-cent call will be a bargain compared with long-distance telephone rates.

When called from any place in the United States, the number 900-410-TIME will give the time from the master clock, actually an array of two dozen atomic clocks accurate to a billionth of a second.

The time required for the time "blip" to be sounded over the telephone wire makes the reading somewhat less accurate but still precise to within milliseconds, officials said.

At a news conference yesterday, Gernot Winkler, director of the Naval Observatory's time division, cited several advantages of the new national 900 number:

* The master-clock number now can handle only 10 calls at once. With the 900 number, it can handle 8,000 calls simultaneously.

* A caller's connection will cut off automatically after one minute, or when traffic is busy, after 15 seconds. The 900 service will allow callers to stay on the line indefinitely at 35 cents per minute after the first minute.

* The 900 service uses land telephone lines, not satellites, to transmit the calls. For those who need very accurate time signals, this is important because sending the time over a satellite link can delay its arrival at the caller's phone.

Inauguration of the new service will coincide Sunday morning with the switch from daylight-saving time in most of the nation and the redrawing of time zones in Alaska to bring most of the state's remote and widely scattered population centers into one time zone.

From east to west, Alaska has four time zones, the Pacific, Yukon, Alaska-Hawaii and Bering.

Under the new system, most of the state will shift to the Yukon zone. The greatest adjustments will be made in the capital of Juneau in extreme southeastern Alaska, which will set clocks back one hour, and in Nome in the northwest, whose clocks will move forward two hours.