Reagan administration officials said yesterday that Soviet leader Mikhail Gorbachev's announcement of a "date certain" for the start of the Soviet withdrawal from Afghanistan was preceded by a spate of private contacts and hints, some of which go well beyond the details Gorbachev made public.

One such Soviet suggestion, which is being taken seriously at the State Department and elsewhere in the administration, is that half of the 115,000 Soviet troops will be withdrawn within 90 days after the pullout begins. That would satisfy the U.S. demand for a "front-loaded" withdrawal schedule.

Another Soviet suggestion not mentioned by Gorbachev is that all Soviet military advisers now stationed in the field with Afghan Army troops will be withdrawn alongside Soviet regulars. Only advisers to the Afghan Defense Ministry would remain in Kabul. This appears to meet another concern expressed by U.S. officials.

Adding weight to Gorbachev's announcement are reports that a drawdown of Soviets in Afghanistan may already have begun. Washington sources said Eastern European diplomats in Kabul are saying that the secondary school for Soviet dependents there is being closed and some Soviet civilian advisers to the Afghan government have been told they are going home.

Gorbachev's announcement, which places his personal prestige on the pullout, was greeted publicly with cautious words from the Reagan administration yesterday. Privately, however, officials said the step may be a milestone because it meets the longstanding administration demand that the Soviet Union set a "date certain" for beginning the pullout to prove it

is serious about leaving Afghanistan.

The timing of the announcement and of related developments, especially between now and the next U.S.-Soviet summit three months from now, is particularly intriguing to Washington officials.

Soviet Deputy Foreign Minister Yuli Vorontsov is expected to bring important details of Moscow's plans when he visits Pakistan on Wednesday, according to Pakistani sources in Washington.

Soviet Defense Minister Dimitri Yazov is reportedly in the Afghan capital of Kabul, U.S. sources said.

With the announcement of a pullout date, conditioned on an accord by March 15 in the U.N.-sponsored Geneva talks, Secretary of State George P. Shultz's visit to Moscow Feb. 21-23 takes on added significance. So does almost three weeks of shuttle diplomacy of U.N. mediator Diego Cordovez, who flew back to Kabul from Islamabad yesterday.

If Gorbachev's timetable holds, the pullout of the first Soviet troops will occur just about the time President Reagan will be arriving in Moscow for a summit meeting with Gorbachev. Shultz said last Friday in Seattle that the summit is likely to be in May, and other officials said yesterday that mid- to late-May is the most likely time for Reagan to be in Moscow.

Senior Soviet officials in Moscow made it clear to recent visitors that they are mindful of the potential importance of their withdrawal from Afghanistan on the Reagan-Gorbachev summit, Senate ratification of the Intermediate-Range Nuclear Forces (INF) Treaty and the prospects for negotiating 50 percent cuts in the strategic nuclear arsenals of the two superpowers.

Reagan, who was asked about the Soviet announcement as he was preparing to return to Washington from an antidrug speech at Duke University, said, "I'm waiting to see what the conditions are."

White House spokesman Marlin Fitzwater, expanding on Reagan's remark, said Gorbachev's statement "sounds like a positive step and we hope it is, but we need to wait and see the fine print."

State Department spokesman Charles E. Redman said, "The establishment of an acceptable timetable for Soviet troop withdrawal has been a key requirement for forward progress in the Geneva process, and we welcome Gorbachev's apparent willingness to address this question."

Redman added that "one key to any realistic and lasting settlement" is whether the millions of Afghan refugees in Pakistan and elsewhere will accept it. He did not indicate how acceptability will be determined. The refugees include members of U.S.-armed Afghan resistance movements, headquartered in Pakistan, which have received more than $1 billion in U.S. aid.

As he has in the past, Gorbachev linked the withdrawal to the cessation of "interference" by the resistance. For the first time, Gorbachev said that is being arranged to "mutual satisfaction." U.S. officials said this suggests Gorbachev expects Washington to accept his withdrawal plan, which would trigger a U.S. commitment of "non-interference" with its implementation.

Staff writers Lou Cannon and Bill McAllister contributed to this report.