In her 1979 collection of essays The White Album, Ms Joan Didion wrote, “Many people I know in Los Angeles believe that the Sixties ended abruptly on August 9, 1969,” the night of the Manson Family murders, when the hippy dream burst. Similarly, for my generation, the 1990s began suddenly, four-and-a-bit months early, on 25 August 1989, the day Bill & Ted’s Excellent Adventure was released and its heroes, Bill S Preston Esq and Ted Theodore Logan, first swaggered out of a time-travelling phone box.

“Strange things are afoot at the Circle K…” Three decades on, Bill and Ted are back. A third movie, Face The Music, arrives this summer and the timing couldn’t be better. First, the world itself could do with a reboot, a recalibration towards the utopian future the first two films promised. And second, that trademark slacker style the pair made their own in the early 1990s is inescapable today.

Back in 1989, Bill and Ted, played by Messrs Alex Winter and Keanu Reeves respectively, seemed to have come not just from another time and space, but from another reality entirely. Their vernacular was slow and stunted, countering “no way” with “yes way”, yet it was littered with flowery, archaic superlatives such as “bodacious”. Likewise, their clothes. They took old-fashioned items such as waistcoats and turned them into something new. And they became a conduit for the Southern Californian style that was about to usurp mainstream youth culture.

“They came about at a time when my teenage self was trying to make sense of it all,” says MR PORTER Style Director Mr Olie Arnold of the films. “I was obsessed with SoCal culture and indie music, so checked shirts, worn open, washed-out T-shirts and Converse were pretty standard outfits for me. I don’t think I fully subscribed to the brighter colours, but the general aesthetic was pretty accurate. I still carry a bit of it today.”

That slacker look – long hair, cut-offs, stone-washed denim, pastel colours, layered tees, band logos, smiley badges, homemade embellishments – had been bubbling up for much of the previous decade, from Spicoli in Fast Times At Ridgemont High to the glut of 1980s teen movies, such as The Breakfast Club, Weird Science and Ferris Bueller’s Day Off. But while Bill and Ted took contemporary tropes from the subcultures surrounding rock and metal, the way they dressed pointed to the future and pre-empted the grunge scene. They changed the sartorial template for the following decade.

Bill & Ted’s Excellent Adventure follows two high-school, air-guitar-playing metalheads with seemingly little in the way of a future. Ted’s dad intends to ship his son off to military school should he flunk out of class (“most heinously”). Only in the future – 2688, to be precise – society itself is built around the teachings of these two vacant adolescents and their would-be band, Wyld Stallyns.

Their destiny, and indeed the fate of civilisation, hinges on a history paper. The leaders of the future have just the thing: a time machine, disguised as a telephone booth, which allows the duo to travel back to the past, hijack historically significant “dead dudes”, including Messrs Abraham Lincoln, Genghis Khan and Socrates (pronounced “So-crates” throughout), and (spoiler) pass the class with flying colours, not to mention Flying Vs.

The film, too, does its history homework. Ms Hilary Davidson, author of Dress In The Age Of Jane Austen, is an expert on Regency clothing. She is also the person behind @BillAndTedTest, a Twitter account that assesses the authenticity of period-drama wardrobes, using the scene where we first encounter Beethoven in Excellent Adventure as its benchmark. “It’s such a small part of the film, but a lovely detail,” says Ms Davidson. “Costume designer Jill Ohanneson pulled from the great stock costume houses in Rome, so there is a careful Italian attention to cut and finish in the costumes, which make use of beautiful satins and laces for evening gowns and suits.”

Ms Davidson thinks Excellent Adventure got its contemporary style right, too. “I remember wearing all that oversized gear,” she says. “A particularly large red and black tartan jacket comes to mind. The stone-washed denim was also a big deal in Australia, where I grew up. I cut off a pair of jeans to make something like theirs. And, like Ted, I had a yen for waistcoats, which I wore over big men’s shirts and flowery dresses. I can’t say I ever thought of them directly as fashion inspiration, but their clothing picks up on so much of the youth zeitgeist around then – big, baggy, layered.”

Excellent Adventure was followed in 1991 by Bill & Ted’s Bogus Journey, in which the pair go to hell and back (yes way). Now, three decades later, they’re getting the band back together, along with Kid Cudi and Arcade Fire’s Mr Win Butler, suggesting they’re transcending musical genres as well as timelines. How the reboot fares is perhaps something the denizens of the future can help us out with, but the fact that Bill and Ted warrant a return after such a long hiatus perhaps says more about today than the past.

Certainly, recent fashion collections have borrowed liberally from Bill and Ted’s heyday. “The 1990s trend definitely reflected that, especially in streetwear brands,” says Mr Arnold. “Lots of pastel-coloured T-shirts and denim jackets and checked lumberjack shirts. Even Rufus’ [Bill and Ted’s guardian from the future] long black leather coat and slim sunglasses came back. Very Balenciaga a couple of seasons ago.”

Mr Arnold notes a marked transition in the wardrobe for the 1991 sequel. “I think it became a bit more directional, less poppy,” says Mr Arnold. “I guess it had to reflect the times and the audience, who were growing up.”

“Bogus Journey’s costumes rode the slacker grunge wave,” says Ms Davidson. “Yeah, we did all wear flannel shirts. Every boy in high school had some variant of Ted’s long hair with an undercut.”

Of course, Mr Reeves is still an icon today, thanks to numerous memes, the John Wick franchise and SAINT LAURENT campaigns, but he defined the 1990s. From the Bill and Ted movies and My Private Idaho (1991) to The Matrix (1999), he bookended the decade’s culture and its style. Neo, his character in The Matrix, brought leather trench coats, pitch-black sunglasses and flip phones firmly into the pre-millennial conversation – and saved humanity in the process.

But why say “whoa” at Neo? Go back further and Ted Theodore Logan is arguably a far better role model. With Bill, he taught us to be excellent to each other and to party on, er, dude. More importantly, he also taught us to learn from the past.