ONE OF the easiest ways to kiss off an old play that seems silly or stodgy or merely strange is to call it "dated." Not bad in its time, you imply, but no longer capable of dissecting the mustard. No offense intended to the author or his heirs.

This has been a temptation lately in Washington, where theatergoers have been exposed to a passel of plays from the Gay Twenties and the Turbulent (but still, theatrically, rather gay) Thirties. The word "dated" has been on many a post-performance lip, particularly after "Daisy Mayme," "Idiot's Delight" and "Home and Beauty," to mention three notably unstirring revivals mounted here in 1979.

But the question -- my question, anyway -- is: Do plays really date?Can the lapse of time turn a good play into a bad one? The answer -- my answere -- is no. Milk dates. Carbon dates.Highschool students date (or once did). Plays do not.

If they seem to, it is an illusion for which there are only two possible explanations: The work was overvalued in its own time, or has become undervalued in ours. It is not always easy, of course, to understand to play that has reached -- as the French say of women -- "a certain age." Rococo language conventions, bewildering religious premises, devalued concepts like chasity or dying of a broken heart, excessive topical references to the politics of a given court and king, coincidences uncongenial to the realistic bias of the post-Victorian mind -- these are just a few of the barriers that can keep us from fully appreciating the theater of another era. But if the reward is, say, a "Romeo and Juliet" -- a play with genuine human interest, dramatic integrity and every one of the outdated notions listed above -- then crossing such a temporal obstacle course can be not only a worthwhile but perhaps even a stimulating passage.

We tend to take such adjustments for granted with plays by Shakespeare and his contemporaries. More subtle problems are posed by more recent plays, English and American, whose characters seem to talk, behave and dress roughly as we do, making it all to easy to judge them by the latest cultural standards. So it is in the great gray period between history and current events -- more than 10 years ago, say, and less than 75 -- that we find the plays that seem to "date" the most.

The hero of George Kelly's "Daisy Mayme" was a middle-aged bachelor who, out of sense of obligation to his widowed mother, his sisters and his orphaned niece, had never married -- never even had a girlfriend. "Is this a problem for our times?" asked one critic with what I took for sarcasm. Well, gosh, actually, yes. The sexual revolution has not wiped loneliness or obsession with one's parents and siblings off the psychological map, even though the manifestations may have changed. If "Daisy Mayme" had treated those questions well -- if it had been a good play in 1926, it would have remained so. Because it was a weak, illthough-out then, with rather pale characters, that's what it was in 1979, too.

"Daisy Mayme" may have held slightly more appeal in the '20s, when its naturalistic approach to everyday middle-class life had a certain novelty value. But even then its appeal was apparently limited, since it closed after 14 weeks on Broadway. Robert Sherwood's "Idiot's Delight" is a clearer case of a play that looked deceptively good in its time. Its stars were Alfred Lunt and Lynn Fontanne, known for their ability to give vehicles the aura of classics. And its dead-ahead look at imminent world war must have lent the play a sense of urgency. At Arena Stage last spring, it seemed neither classic nor urgent.

The current Arena production of "Design for Living" has aroused astonished comment (even from some who find the performance less than enthralling) about how little the play has dated since 1933, when Noel Coward wrote it. Defending her unmarried state, the heroine explains, "The only reasons for me to marry would be these: To have children; to have a home; to have a background for social activities and to be provided for. Well, I don't like children; I don't wish for a home; I can't bear social activities, and I have a small but adequate income of my own. I love Otto deeply, and I respect him as a person and as an artist. To be tied legally to him would be repellent to me and to him, too."

A woman of 1980 could say something similar. But that's not why "Design for Living" seems livelier than "Daisy Mayme." It seems so because it is, and always has been. The virtues of "Design for Living" -- its wit, polish and driving energy, all harnessed to serve the idea of irrepressible, three-cornered love-transcend the advanced morals of its heroine.

"You Can't Take It With You," the other half of the Christmas bill of fare as Arena, comes from the same era (raising a tangential question: Are modern playwrights incapable of writing for the holdiays?). Like "Idiot's Delight," George S. Kaufman and Moss Hart's play reminds us the Pulitzer Prize was no surer measure of deep worth in the mid-'30s than now. Many of the gimmicks the authors jammed into their concoction were slapdash indeed, and have grown more so through over-use in subsequent stage, movie and TV comedies. ("Arsenic and Old Lace," for example, also had a bizarre family with one normal offspring, criminal mischief in the basement, and a possible marriage in the balance. And how many times have the families in TV sitcoms been subject to police raids because of some innocent but misunderstood action of an eccentric family member?) But while the play may not have anything much you can take with you, it can make an audience laugh -- still. Go. Try not to laugh. Consider yourself dared.