“So. What’s the next book going to be?” I really didn’t have an answer. I’d stumbled a little as a TV screenwriter — a returning series that had failed to return — and so had turned to writing fiction and had promptly stumbled there too. My first novel, Starter for Ten, had done well but, like many debuts, had used up a lifetime’s worth of jokes and stories and the follow-up, The Understudy, had not come as easily. If second novels are famously hard to write, then a third seemed impossible. I thought I might write a love story this time, “something epic”, but that was as much as I had.

Thankfully, Tess of the d’Urbervilles came along, a dream television job, adapting a book that I’d loved as a teenager, tearing through all of Thomas Hardy’s novels, ostentatiously moved by the fate of poor Henchard, poor Jude, poor Tess. In our BBC production she was played beautifully by Gemma Arterton, with Eddie Redmayne as her Angel; here was a proper love story, an epic.

Yet still there was that novel to write. I’d recently turned 40 and become a parent and so was much obsessed with the passage of time. A father, a writer — neither of these things would have seemed plausible to my 20-year-old self and perhaps there was something in that journey from there to here, the people who change the course of it. It’s an idea that Hardy comes back to repeatedly; if only Tess’s letter had not slipped under the doormat, if only Angel had danced with Tess on May Day. A younger generation thinks of these moments as “canon events” and to one, possibly two generations above they’re “sliding doors moments”. For the reader and viewer there’s a particular pleasure in observing these turning points in a way the characters never can — frustration, melodrama, pathos too, those scenes where you want to climb inside the story and say, “Oh God, don’t do that.”

The Idea

That’s still not enough for a novel, though, and I don’t think I had it in me to write one of those whole-life epics. In adapting Tess I’d been reminded of a famous passage that had obsessed me as a teenager, the moment in chapter 15 where she looks in the mirror and wonders if that date might have a significance that she is yet unaware of, a kind of hidden anniversary of future events, “a day that lay sly and unseen” in the calendar. Perhaps this was the answer, to use an ordinary day as a container for the whole year’s events, to take 20 snapshots that tell a larger story, only revealing the significance of the day at the end. Twenty birthdays wouldn’t do it and twenty Valentine’s Days or New Year’s Eves would soon lose their charm.

Neither could the ordinary day be too ordinary, no boring old February 6 or November 2, because the characters would need to notice it, remember sometimes but not always. Graduation, or perhaps the day after, seemed like a good place to start the journey into adult life, and there was St Swithin’s Day, a kind of Chaucerian Groundhog Day, a tradition that’s all about our desire, and inability, to predict the future. “St Swithin’s Day” is the title of a lovely Billy Bragg song that I’d listened to a lot at 20, all fading Polaroids and lost love, and St Swithin is buried in Winchester, not far from where I grew up and also the city where poor Tess meets her fate. July 15, but told 20 times; that might work.

The Planning

Planning St Swithin’s Day (working title) took some time. Each ordinary day had to have a hook, an event, a chance encounter or conversation that might have some significance or echo many years later. Neither could the days be too eventful, though the finished novel pushes its luck in this regard. I still have the chapter breakdown for Twenty Years (another working title), which consists entirely of “Tom” (a “men’s magazine journalist”) and “Kelly” (“some kind of entrepreneur?”) meeting repeatedly at parties, at which he’s drunk and she’s sarcastic and yearning for him for no discernible reason.

The shape is there, but it’s incredibly boring and repetitive and clearly the story would need opening out, embellishing with some comedy and suspense if we were ever to care about Tom, later Will, and Kelly, then Maeve. Other characters appeared — the failed stand-up, the married man, the parents — and as the synopsis became more populated it became easier, necessary in fact, to give the lovers — Emma and Dexter at last — separate lives, to find ways to keep them apart. For perhaps two thirds of the finished novel they’re not even in the same room, and there was a useful tension in this, building that desire to finally bring them together.

The Writing

Once planned the novel was written relatively quickly over nine months, then written again, retyping from a printed manuscript. I don’t recall much about the process except that it was fun, or as much fun as it can be to sit by yourself and type. I’m always skeptical of the idea of characters “coming to life”, “becoming real” or “taking over”, because writing is about construction, but I did grow to know them better, to love them and to be maddened by them, in the same way that our best friends can sometimes frustrate us without ever jeopardizing the friendship.

While it’s by no means an autobiographical novel, little bits of memoir inevitably slipped in, particularly in Em’s half of the narrative; the terrible restaurant was pseudo-French not Tex-Mex, but I’d sat on Arthur’s Seat in the summer of 1988, started writing in Paris in 2001, taken my first foreign holiday in Greece in 1992. Emma’s surprise at seeing a lemon growing on a tree was my own, as was her sense of panic and uncertainty at 21.

Dexter’s hedonism was certainly not my experience of the Nineties, but I’d watched my contemporaries have a lot more fun than me, then struggle to keep it up as their twenties faded away. I listened to the music from each year, read the headlines and — more usefully — the TV schedules, peered into old photographs until I thought I could remember what it felt like to be 21, 26, 34, note what changes with time, what stays the same. The novel was finally published in May 2009, changing my life in ways that my younger self would have struggled to comprehend.

The New TV Version

Which brings me back to time. I’m closer to 60 than 50 now, and the baby who cried while I wrote the first draft is now thinking about university. In books and on TV I consume young writers’ coming-of-age stories set in the 2000s and 2010s and I think, hang on, that’s yesterday. Time accelerates and I struggle to understand how there can be 15 years between “Love Me Do” and the Sex Pistols, but nearly 30 years since the release of “Common People.”

As I write this we’re preparing to launch a new screen adaptation of the book that takes full advantage of its episodic nature, one episode per day, starring two wonderful, charismatic young actors who weren’t even born until halfway through the action, so that this story bears the same relation to them as the Suez Crisis does to me. My youth has become a period drama, and their “Blue Monday” is my “Rock Around the Clock.” That can’t be correct. I’ll do the sums again. No, that’s about right.

Like photographs and diaries, the stories we tell belong to the time in which they’re told — how could they not? — and working on this new version as an exec (so grown-up), not a writer, has sent me back to the novel and the things that I couldn’t or wouldn’t write now. The absence of technology is striking of course — how could you possibly “lose touch” with someone now? — and the novel is noticeably coy about sex, which is strange given how powerfully it motivates the characters.

Then there are the jokes that don’t land, an excess of pop-cultural references, moments that are overstated or thrown away. Like a diary, like an old photograph, I flick through and wonder, “What were you thinking?” I still love Hardy, although I’m less taken with the doomy fatalism that I adored as a teenager and I can’t claim that there’s anything particularly Hardyesque in the novel — no great poetry, no landscapes — except perhaps a certain morbid romanticism, momentary decisions that change a life’s course, for better or worse.

The lost letter that Dexter writes, inviting Emma to join him in India, is stolen from the note that Tess inadvertently slides under Angel’s doormat, and there are small thefts from Far from the Madding Crowd and Great Expectations too, books that I have since had the pleasure of adapting for the screen, but which belong to a different part of my life, like an old pop song that I still love, although more in the spirit of nostalgia.

Like photographs and diaries, the stories we tell belong to the time in which they’re told.

The writers I read at that time of life, from 16 to 22, Salinger and Scott Fitzgerald, Dickens and Hardy, still linger somewhere in almost everything I write. Take this passage from Great Expectations that Emma quotes in our new version: “Pause you who read this and think for a moment of the long chain of iron or gold, thorns or flowers, that would never have bound you, but for the formation of the first link on one memorable day.” It’s as good a synopsis as any, although in Em and Dex’s case the chain is largely gold, mostly flowers.

But I’m still very fond of the novel, proud and grateful for its reception. I’ve written six novels altogether, but when people occasionally say, “I enjoyed your book,” I know which book they mean. Readers still say that they see themselves in Emma, and see their friends in Dexter, and that means a great deal, that sense of identification. And yes, it is an epic love story, but revisiting it now it seems to me to be a book primarily about friendship, about our capacity to change each other’s lives for the better through conversation and care.

And for all their flaws, I do still love Em and Dex, especially in their new incarnations. The screenwriters and the performers too have given them a wit, an intelligence and charm that was not always on the page, and their charisma seems extraordinary to me. I still love them, although they sometimes frustrate me, as good friends will do. And it does feel epic, both in its styles and settings — Paris, Greece, Rome, London, Edinburgh — and in its emotions. The original intention was to write something that felt like a great pop song, something elating, joyous and sad, constantly shifting between major and minor keys. That was 18 years ago and it’s such a thrill to see it coming alive again after all this time.

One Day is available for streaming on Netflix

David Nicholls is an English novelist and screenwriter