One Day shouldn’t work. Consider its premise: posh southern boy fancies working-class northern lass … for two decades. He grew up in the Cotswolds and is planning on spending the summer after graduating from university in France “with the Marlborough lot”. She tours dusty village halls with plays about suffrage and pauses to put on Joan Armatrading’s Love and Affection before she gets off with someone. It doesn’t sound plausible. Or touching. Or funny. Or relevant to our times, which have travelled so far beyond “class-conflicted” that I can’t summon a word vicious enough for them.

And yet. When David Nicholls’ third novel came out, in 2009, it won over every person who read it, even the ones who don’t do romantic comedy. It was so funny and unpretentious, so heartfelt and true. The film that followed was weirdly awful, but let’s treat that as the rebound relationship instantly excised from your romantic history so you can love again. Because we now have the hot 2020s version. And this one’s a keeper.

A limited series is the perfect format for One Day, a love story at once epic – spanning 20 years – and quotidian, with all the action taking place on a single date, 15 July, St Swithin’s Day. Each half-hour episode, bar the heart-wrenching finale, covers a day, beginning in 1988 with Dexter Mayhew (Leo Woodall) and Emma Morley (Ambika Mod) meeting on the night of their graduation from Edinburgh uni. This neatly carves up the story into 14 highly bingeable slices of wistfulness.

The format made me realise what One Day is really about: the poetry of day-to-day life and the power of nostalgia. As Philip Larkin wrote in the poem that opens this flawless adaptation, written by a team led by Nicole Taylor (Wild Rose) and executive-produced by Nicholls: “Where can we live but days?”

Each episode is an exquisite miniature set-piece. There is the 15 July when Dexter, now an unravelling TV presenter, goes home to visit his dying mum and screws everything up. Even the abandoned exercise bike in his old bedroom and the Bugsy Malone video on the shelf bring a lump to my throat. And the woozy summer’s day when Em and Dex get a bottle and a bag of Kettle Chips from the off-licence, lie on Primrose Hill and watch the sundown. And the excruciating round of the parlour game Are you there, Moriarty? when Dex accidentally whacks his new, even posher girlfriend in the face with a rolled up copy of the Times.

The supporting cast is superlative, the period details magnificent, the soundtrack sublime. By the time Rip It Up by Orange Juice comes on, I am a mess, capable of falling apart at the sight of a hamburger phone.

Of course, the whole enterprise hangs on Em and Dex, who must be completely believable and lovable, individually and together. They are extraordinary. Woodall’s wide-boy charisma and frightful yet endlessly forgivable privilege are perfectly pitched; I forgive him a thousand times. His poshness is neither glossed over nor glamorised; it is simply integral.

Ambika Mod (This Is Going to Hurt) is such a revelation that it is hard to believe this is her first lead role. Her Em is flinty, vulnerable, committed and relentlessly deadpan. On the hungover morning after they meet, they pass the time climbing Arthur’s Seat. “Is that a religious thing, not sleeping together?” Dex asks. To which Em explains drolly that her mum is a Hindu and her dad a lapsed Catholic: “God wasn’t involved.”

Which brings me to a confession. My initial reaction to seeing Mod play Emma was complicated. I was thrilled: I’ve never seen someone who looks like me, living the life I lived around the time I lived it, on screen before. But this is also why it didn’t ring true. Because race (and class, for that matter) didn’t work like that back then. By which I mean – I’ll just say it – white boys like Dex didn’t fancy brown girls like Em. I know because I was one, although I went to Glasgow. In the first few episodes, the unmentionability of Emma’s race got to me. The truth is, Dex (and his parents) would have made unintentionally racist blunders. And Em would have forgiven him.

But then I kept watching. I started to fall for them, hard. And I began to appreciate the lovely Shakespearean bones of this story, which is about two wildly different people connecting through humour, shared cultural references and, above all, time. And, God, it passes so fast. Suddenly it is 15 July 1991, Dex and Em are skinny-dipping in Greece and I am all in. By the time it is 2007 and Dex is back in Edinburgh, where I ended up and from where I am writing these words, I am crying my eyes out and longing to do it all over again.

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