For the last century, military personnel have been paid on the last day of the month. This month, thanks to a bit of congressional budget-cutting gimmickry, the 2.1 million men and women on active duty are going to have to wait an extra day.

The wait will save the Defense Department, on paper, nearly $3 billion in fiscal 1987.

Under a provision included by Congress last fall in the fiscal 1987 budget, the official military pay date is about to change from the last day of the month to the first day of the month.

Instead of getting paid on Sept. 30, members of the armed forces will have to wait 24 hours until Oct. 1.

Those two dates just happen to mark the end of fiscal year 1987 and the start of fiscal 1988. And therein lies the gimmick.

By ordering the change to become effective this month, Congress was able to shift the nearly $3 billion in Pentagon salary expenses from one fiscal year to the next, providing a relatively painless means of cutting the Pentagon's budget request without really cutting it.

The Defense Department wanted to leave things the way they were, but now has no choice but to comply with the congressional directive.

Congressional aides who helped fashion the Pentagon's budget last year freely acknowledged the modification was nothing but a gimmick, available for use one time and one time only. Because the change is permanent, the 1988 fiscal budget will also cover only 12 pay dates.

As the Pentagon prepares to make the switch, it has had to grapple with one problem that Congress never addressed: Does the directive apply to the hundreds of thousands of men and women in the reserves and National Guard?

The short answer, as finally handed down by the Pentagon's Office of General Counsel, is no.

The Army in particular had fought for that interpretation, arguing that pay schedules for reservists and guardsmen would be seriously disrupted if the change were applied to them.

The Army saw no reason to postpone such payments until the first day of the next month, and the general counsel's office, after reviewing the law and its legislative history, concluded the statute was aimed only at active-duty forces.

While the change in pay dates saves a lot of money in fiscal 1987, the move is not without cost. Although no one has ventured to put an estimate on the tax loss, active-duty servicemen will get a one-time tax break in exchange for putting up with the one-time shift in pay dates.

The reason: personnel won't receive a pay check on Dec. 31 this year, pushing that portion of their 1987 income into the next tax year.