Jane Austen was no slouch when it came to the coining of pithy catchlines. You don't have to stretch the fancy very far to imagine a cinema poster for Pride and Prejudice adorned with "It is a truth universally acknowledged that a single man in possession of a good fortune, must be in want of a wife". Well, yes, all right ... that might prove a little too elegant for contemporary tastes. After its passage through the colonic bureaucracy of the marketing department it would probably look different - "He's loaded. He's loathsome. He's looking for love", perhaps, or "He's got the means but is he too mean to marry?". The point is that Austen would have understood the impulse to pitch the essential core of the story to the audience. But even if she was broadminded about the cinema's commercial impulses, I think she might have taken a slightly giddy turn at the sight of the catchline for the recent film of Emma: "This little cupid's no angel", if my recollection is correct, a piece of copy that accompanies a winsome photograph of the film's star, Gwyneth Paltrow, looking as puckish as she can manage.

This isn't the only pitch made to potential ticket-buyers. Both the trailer and the poster use the word "timeless" to refer to the story, a word also employed in the trailer for a forthcoming movie version of Jane Eyre. Obviously, "timelessness" is deemed to be a tempting commodity for cinema- goers, which is slightly paradoxical when you consider that time, above all, is what a period film offers you. In one sense, these films aren't timeless at all - they are time-crammed, bulging with history, or what passes for it in Hollywood (long dresses and nobody saying "yeah"). Wouldn't it be more logical to trumpet these virtues - "Jane Austen's wonderfully dated story", say, or "as old-fashioned as they come", particularly because such films (quite unlike the novels on which they are based) necessarily exploit a nostalgia for vanished manners and modes of life. When you read Emma, the past has no opportunity to impress you as an escape from the present - because unless specifically mentioned, the paraphernalia of period is effectively invisible. When you watch Emma, however, everything must be present and correct, and the negligible elements of any scene - the shape of the tea-cups or the cut of a bodice, jostle for your approval. We watch in large part because we desire to be out of our own time, and anachronism - which might be the best demonstration of a work's genuine timelessness - is perceived as a breach of contract (those who talk about the "timeless" qualities of Shakespeare are quite often the same people who moan grumpily about modern dress productions).

Naturally, the attention we pay is highly selective - the implicit presence of modern dentistry and modern detergents would only be troubling to the most determined refugee from the 20th century. And while a film based on the life of George Washington might be brave enough to include outdated racial opinions, or scholarly enough to relish the accuracy of a frock coat, it would almost certainly draw the line at exposing the President's wooden false-teeth. In a similar vein, as Adam Mars-Jones pointed out on our Film pages last week, every director of an Austen adaptation has to decide whether servants are a pleasing accessory to the dream or an embarrassment we don't quite know how to cope with.

The copywriters, I would guess, don't have such matters in mind. They are simply using "timeless" as a euphemism for classic, a sort of chronological superlative - the word is a shorthand for saying something like "so good that the passage of time has had no effect on it", as well as offering a mild reassurance that you'll still enjoy it. But even here the phrase raises some questions. For a species that can never quite forget its own mortality, the idea that something is exempt from the fatal tick of the clock has an obvious appeal. Jane Austen, the woman, wasn't timeless but her comedy has proved itself so; perhaps we should buy a ticket in the hope of getting in on the secret. But what would it be for a work to be genuinely timeless, to pass through the years without any abrading friction whatever, so that it registered on an audience in 1996 exactly as it had a hundred or even 500 years earlier? Is such a work even conceivable?

"Timeless" doesn't worry about such issues because it has simpler business in hand, encouraging cinema-goers with a coded guarantee of antiquity. Strictly speaking, a work written last year might be equally "timeless", equally resistant to temporal decomposition, but it would never be described as such because it would be a false bill of goods. "Timeless" actually means "timeful", and you only achieve that by waiting patiently.