At lunchtime on 31 December 1999, our phone rang. I answered.

“Is that Conor?”

I knew the voice immediately. I swallowed and said, “It is.”

He said that they were driving from south Dublin to the family home in Derry for the millennium New Year’s Eve. We were living pretty much on the Irish border at the time. He wondered if they could break the journey by stopping at ours. They were just getting in the car. I, playing it breezy, said we’d be around all day and

be fine.

Our son was four, our daughter three. They were in their pyjamas in cardboard boxes on the kitchen floor, playing dodgems. Their mother was up to her elbows in baking.

I said, “Mr and Mrs Seamus Heaney are driving north for the new year.”

She said, “And?”

I said, “Can they swing by for a cuppa?”

And she, considering the general state of the place, said, “Jesus, Mary and Joseph.”

The Heaneys had always been incredibly friendly and kind to us, a couple of young poets. Seamus had given a reading of his poetry locally a few years before, just after he had won the Nobel prize. He and Marie, his wife, had come out for a meal afterwards. But we were too starstruck to presume to call them friends.

We bathed the children and dressed them in their Christmas best, scrubbed the house and ourselves, lit a fire and dusted down an ornamental teapot we had never actually used. We even had enough time to twiddle thumbs before their bottle-green Merc drifted up and we, indoors in half-light, rose to our feet.

Nice story, isn’t it? It has a pleasing coda: the Heaney collection of 2006, District and Circle, has a gorgeous poem called Midnight Anvil about the millennium New Year’s Eve in rural Derry and the sound of an anvil hammered by the poet’s uncle echoing all over the parish. Every time I have told this story I have ended by invoking Midnight Anvil and saying: “See that poem? They stopped off at ours on the way to that poem!”

But here’s the thing: it may not be completely true … I just opened my copy of District and Circle. It’s been ages since I read it. To my horror, the poem seems to suggest that its author was not, in fact, in Derry on that famous night. Its opening, for example, goes:

If I wasn’t there

When Barney Devlin hammered

The midnight anvil

I still can hear it: twelve blows

Struck for the millennium.

Oh dear. Did they not go to Derry to ring in the millennium? Did they call on us on a different New Year’s Eve?

We don’t live on the border any longer. Both of our children have grown up. Myself and their mother parted ways 10 years ago, and the great man – it scarcely needs saying – has gone into what metaphysical poet Henry Vaughan called “the world of light”.

But let’s not turn gloomy. Our kids are healthy and doing well. Their mother and I remain on good terms, and the four of us still spend Christmas together. The day Seamus died, in August 2013, I was in the old Spanish city of Girona. I called my ex-wife when I heard, and one of the first things we both remembered was their visit to our home one New Year’s Eve.

Besides, if Midnight Anvil is about anything, it is about those things we think we remember even though we are no longer entirely sure that we were there.

You want to know what happened that afternoon on the eve of the new millennium? Not very much. They came, were lovely, sat half an hour and drank tea. If I remember rightly, we discussed the Y2K bug (which Seamus declared “a cod”), Nostradamus and the possibility of our universe imploding that very night. The ornamental teapot wasn’t a great pourer. After a fashion, their bottle-green Merc drifted onwards, north across the darkling border, and our little son asked: “Can I take off these bloody clothes?”

I used to think change took place in big, dramatic chasms you could point to retrospectively and say with certainty, “It was then that …” I know now it happens mostly in infinitesimal shifts. We don’t see it at the time. We just recognise, looking back, what it was. For hours after they said their so-longs, our still-intact universe felt huger and more starry and, somehow suddenly, nearer.

• Conor O’Callaghan is an Irish writer based in Sheffield. His most recent books are the novel Nothing on Earth (2016) and the poetry collection Live Streaming (2017)