President Corazon Aquino on Wednesday set her nation on a new collision course with the United States over strategic military bases, publicly differing over the expiration date of the treaty covering the installations.

U.S. forces must vacate Clark Air Base and Subic Bay Naval Base by Sept. 16, 1991, unless a new treaty is in force by that date, Aquino said in response to written questions from reporters.Her stand contradicts the U.S. position that the treaty can only be abrogated with one year's notice and that such notice may not be given until September 1991. That would make the earliest date for the departure of American forces Sept. 16, 1992.

Philippine Foreign Secretary Raul Manglapus, who last month signed a two-year pact covering the operation of the bases through 1991, admitted in an interview last week there was some question about the expiration date of the treaty, which has been in force since 1947.

An amendment signed in 1966 specified the treaty "shall remain in force for a period of 25 years from Sept. 16, 1966, after which, unless extended for a longer period by mutual agreement, it shall become subject to termination upon one year's notice by either government."

U.S. officials believe the language makes it clear that notice of termination cannot be given before 1991, but several Philippine senators have begun pressing the government to give notice in 1990, arguing the treaty may then be terminated in 1991.

The senators also note a clause in the 1987 Philippines constitution saying that "after the expiration in 1991 of the (bases) agreement, foreign military bases, troops or facilities shall not be allowed in the Philippines except under a treaty duly concurred in by the Senate."