Writers and publishers, like other manufacturers, can be opportunists, and many believe themselves to be on the brink of a particularly juicy marketing moment. Throughout 1999 and 2000 we will be encouraged to look back exhaustively at a decade, a century and a millennium. But, as at the end of each of the decades we have so far survived, journalistic retrospect is also a way of drawing a line under a period, summing it up prior to moving on. The writers of the entre deux guerres and of the war itself got forgotten after it was over: another new beginning had arrived, and several of those writers have still to be recovered. A century's end can seem to end substantial work in progress, but a hunger for new beginnings has nothing to do with the way poetry actually works.

When Thomas Hardy in spectral mood heard 'The Darkling Thrush' on 31 December 1900, he recorded frost, Winter's dregs, day's weakening eye, bine-stems like broken lyre strings; all mankind that 'haunted nigh' were beside their fires, ghostly. He saw nothing less than 'The Century's corpse outleant', not etherised upon a table but in a crypt, the wind registering a death lament.

The ancient pulse of germ and birth
          Was shrunken hard and dry,
And every spirit upon earth
          Seemed fervourless as I.

The thrush he hears is aged, frail, gaunt and small; yet he (the bird is gendered) utters 'a full-hearted evensong/Of joy illimited'. Hardy infers 'Some blessed Hope, whereof he knew/And I was unaware.' Hardy was sixty. How was he to know that of the 887 pages of his eventual Collected Poems (he died in 1928), 750 were still to write? The Hope, he believed, was not for him: he was a spent force. Interesting that he capitalises Hope and has the thrush singing 'evensong' - his usual, unsatisfied metaphysical cravings? Or a flickering belief that Grace existed, even if not for him?

The much-publicised reality of our approaching major calendar change is the Millennium Bug whose repercussions may be devastating. But the black magic of dates is another matter and should be more manageable, though I doubt that it will be. The turn of a decade always seems more important than the nine year-turns that lead to it. Centuries and millennia, how much more so.

In the world of poetry, 'periods' are marked by anthologies, and the first two significant volumes in what will doubtless become a library of interpretative summations have been published this autumn, The Penguin Book of Poetry from Britain and Ireland since 1945, edited by Simon Armitage and Robert Crawford, and The Firebox: Poetry in Britain and Ireland after 1945 (Picador), edited by Sean O'Brien.

Reviewers have concurred on the dumbed-down quality of the introductions to both volumes, especially the former, and on the incoherence of poets chosen and omitted, poems chosen and omitted. Here is something more troubling than the articulate storms provoked by publication of the MotionMorrison Penguin Book of Contemporary Poetry (1982) and the HulseKennedyMorley The New Poetry (1993). The first had an agenda, even if a reductive one, outlined in a comprehensible introduction; the second, regardless of its hectoring and assertive editorial tones, provided substantial selections sometimes well-chosen. There were omissions of course: that is the nature of anthologies. But there were suggestive inclusions.

The two new anthologies are similar to one another in perspective and in their levels and qualities of tolerance. Tolerance is the last virtue one seeks in a dedicated anthologist. O'Brien is less fashionable in his choices, more clear-headed in his politics, and more useful in his selections. But neither volume (and both are vast) gives a sufficient body of work by any single poet, even the better known, to allow a new reader to form a judgement. Most poets are represented by one or two pieces, some by extracts. And the poems chosen are often, it might seem to those familiar with a poet's oeuvre, ill-chosen, designed to adjust a poet to an editorial template, or drawn out of a bran tub.

This kind of anthology has had its day. In the first place, the arbitrary starting point 1945 omits - draws a line against - the poets of the Second World War, some of them younger than poets actually featured, but who had the misfortune to die too soon for consideration. I am not persuaded that Hughes, Tomlinson and Hill make full sense, in the post-1945 period, without Keith Douglas, who is doomed, along with Alun Lewis and Sidney Keyes, to miss the feast for ever. In O'Brien's volume poets who survived the First World War are included; those who failed to survive the Second are omitted. The natural starting point might have been with the Modernists; or if not with them, then with Empson and his generation. Not events, but radical changes in the art which begin rather inconveniently in calendar terms, ought to mark starting points.

I am not sure that Hughes and Plath (Plath here a British subject, along with Sujata Bhatt who has never even resided in the UK) make full sense without Roethke, or Lowell; or that Hill comes clear without Allen Tate. If it is Britain and Ireland, on what logical grounds (other than the usual cultural condescension upon which some Irish critics have remarked, that odd country and Ireland) can the other English-speaking nations be excluded? It is a convenience, of course, but a diminishing one because in not finding space for major English-language poets of the period they leave substantial space for strictly local work.

It is out of the awkward juxtaposition of major poems and strictly local work that the flat, unfocused critical prose of the introductions, the tendentious polemics about democracy, pluralism, vitality, 'niftiness' emerge. William Scammell, reviewing the new anthologies in the Independent on Sunday, declares of the Penguin book, which quotes in order to 'refute' Adorno, 'What Auschwitz is doing here, amid all these trendy mantras, is hard to say. It suggests a crassness of judgement, a fake gravitas, that is echoed in such hollow formulations as "the community of democratic voices".'

The fact is that there are several outstanding Irish and British poets in the last half century (not all of them represented in these books), but not 170 who belong on stage with the outstanding English-language poets of other lands. The absence of Ashbery, Bishop, Curnow, Murray, Prince, Judith Wright and a couple of dozen others is as damaging as the presence of the local work included. How do we take a true measure of the mature Heaney in the absence of Lowell and Walcott? Geography and nationality are two of the things Yeats suggested people talk about when they pretend to be talking about poetry. What matters is not gender, ethnicity, passport or date of death: what matters to the reader of poems and to the poet is what is happening in the language. What should mark the anthologist is a vocation for finding work that challenges and extends the tradition, as much as possible in blithe disregard of the parameters a publisher has set.