Fashions Never Change

Alina: Yep, I think they're time travelers.
Kimiko: You think so?.

Alina: If the future did a documentary of the last fifty years, this is how badly the reenactors would dress.

—Dresden Codak

A standard simplification used in shows set in a particular time period, as well as Speculative Fiction involving Time Travel. Despite the radical way fashions change over short spans of time—compare the 1950s to the 1960s, say, or even the 1970s before and after the advent of disco—anything set earlier than the mid-20th century assumes a generic fashion style that has little variation. Thus there's "standard Victorian dress", or else a typical "Renaissance costume", even though the Renaissance lasted over three centuries. The further back you go, the more generic it gets, with the same "Middle Ages" clothing being worn whether it's the year 800 or 1400 (and whether it's in England or France). Then there's "Ancient Rome"...

The main reason for this trope is that people think that fashions didn't change from season to season until the rise of the middle class in the 20th century. This is not the case; there's a lot of evidence showing that fashion has changed with the seasons in Western Europe since at least the 12th century and possibly much earlier. There have always been people with enough money to spend on new clothing every year, and there have always been fads. Samuel Pepys writes in his 1660s diaries about how both men's and women's fashions changed so quickly he could hardly keep up—and because how he and his wife dressed really mattered with respect to him being thought genteel enough to hold an important post, it wasn't something he could afford to ignore, either.

Of course, some fashions don't change so quickly; jeans and a T-shirt have survived basically unchanged since the end of World War II (though the number of situations in which it is acceptable to wear them has increased), and tuxedos (the most formal version, at any rate), along with the suit-and-tie, have been around for over a hundred. Ceremonial garb, such as the "scholarly" robes you see at universities, is similar to what was actually in fashion for scholars 1,000 years ago when the first universities were founded. Likewise, many religious orders wear centuries-old fashions for special occasions, and in rare cases all the time.) But when you have details that any real-life native of the time period would gawk at, you're doing something wrong.

A frequent component of Hollywood Costuming. Compare No New Fashions in the Future.

Examples of Fashions Never Change include:

Anime and Manga

Just about any giant robot anime predating Macross includes disco haircuts and collared jumpsuits.
One particular offender is the Gundam franchise, where each series generally dresses its characters in fashions resembling the decade in which it was made. For example, Gundam ZZ is so totally The Eighties.
Gundam SEED manages to avoid this most of the time. When we see characters wearing civilian clothes they're usually wearing what appear to be original, or at least uncommon fashions. The problem with this is that the fashions are often rather bizarre & impractical. For example, when we first meet Kira, he's wearing a black shirt that appears to have had the sleeves cut off at different lengths & then reattached with red leather straps & has a hole cut into the chest held closed with more tiny straps for no apparent reason.
- Kira was firmly established as a pretty snazzy dresser throughout both series, and his shirt appears to be based on a traditional shirt of Japanese shrine maidens, with their detached sleeves held together by red ribbons.
Code Geass manages to avert, play straight, and play with this trope in multiple ways. The main cast usually avoids this altogether by wearing either their school uniforms or their Black Knights outfits. In some cases, they can be seen wearing either "modern" (by our standards) or completely original fashions (much like SEED). The problem with that becomes apparent when you remember that the Geass-verse exists in as an Alternate History to our own and that the equivalent to 2017/18 A.T.B. is roughly 1963 A.D. Thank you, Alien Space Bats. This is played with (and possibly straight) even further when you get a good look at the fashions of the nobles and the Britannian Imperial Court: their fashion tastes seem to have not changed since the Golden Age of European Absolutism... which is actually very fitting... There's no excuse for Charles' epic curls.
- Though some of the more formal civilian outfits do look reminiscent of the early '60s.

Comic Books

There is a Disney comic that parodies The Lost World, set in an Alternate Universe and featuring Scrooge, Donald and Fethry. The interesting thing is that the three are wearing their "normal" clothes (Scrooge's top hat and frock, Donald's sailor outfit and Fethry's sweater and signature cap) while everybody else is wearing Victorian era fashion. Stories set in some kind of alternate past are common in Disney comics, but basically all others have the main characters appear in the appropriate historical clothes.

Film

Also parodied in Back to The Future III, when Doc Brown dresses Marty up in a vintage-1955 TV cowboy costume (bright red and pink in color) before sending him back to The Wild West; Doc assures him that he'll fit right in; after all, that's how cowboys dressed back then, right?
Averted in both Film versions of The Time Machine: one way the passing of time is shown is that fashions in a shop window change.
Directly subverted Somewhere in Time. Christopher Reeve's character (Richard Collier) has enough foresight to buy a suit from a vintage clothing shop before heading back in time. When he gets back to 1912, he discovers that he is wearing a suit that was in fashion two years before and he sticks out like a sore thumb. Fortunately, the object of his journey finds this goofiness charming.
The Village is supposedly set in 1897, and features a Pennsylvania farm village where everyone dresses like it is 1797. Possibly deliberate because it is actually the 20th century. Perhaps the Village's founders discovered it was easier to stay 100% self-sufficient with 18th century technology, but their historians knew more about the 1800s and based the community's backstory on that.
Anton Corbijn's Control gets all the changing 1970's fashions right. So the Ian in 1973 goes to see Bowie with guyliner and a fluffy jacket, but by 1979 is wearing the familiar austere Joy Division outfit.
Averted in A Knight's Tale, where our hero was laughed at for wearing old-fashioned, out of style armor in his first tournament.
Averted in The Magnificient Ambersons which opens with a montage of a character wearing various fashions that appeared over the past few months.
The World War II soap opera "In Harm's Way" (made in 1965) is particularly bad with this. All of the women have mid-sixties hairstyles and dresses. Also, much of the military equipment used is of 1960's vintage.

Literature

The Time Scout novels very deliberately averted this trope by suggesting that time travel is actually very dangerous and requires meticulous research, because showing up wearing the wrong shirt collar could prove fatal when one deals with superstitious, xenophobic natives.
Played with in Neal Stephenson's novel series The Baroque Cycle, which shows it works both ways: even though it's set in the age of periwigs and weskits, The Confusion briefly mentions old-fashioned characters who haven't got the memo and still wear ruffs and pointy beards. Because fashions don't change uniformly.
- And then when the style of wigs changed, characters who hadn't been in Europe for years didn't get the memo either, and it was mentioned that the new styles wouldn't be recognizable to them.
- Also largely averted, with much made of the different fashions that the hoi polloi wear at any given point. Stephenson usually does the research.
Parodied in Pyramids, where the Tsortian envoy to Djelibeibi dresses in a mishmash of garments from half a dozen periods of that nation's 7000-year history. A footnote compares this to wearing a mix of old Celtic, medieval, and modern British garments to pass as an Englishman.
Played with in Diana Wynne Jones's Hexwood, where Vierran, the girl providing costumes for intergalactic travellers, has a puckish sense of humour. The Reigners end up with a business suit (Reigner One), New Look-style ladies' suit and high heels (Two), outfit that "looks like Superman" (Three, but then he didn't bother to ask for a costume), ill-fitting Norman Wisdom suit (Four) and monk's robe (Five). Vierran gives herself authentic 1990s casual clothes (jeans and long jumper) and Two tells her "You look like a peasant from New Xai".
Averted in Michael Crichton's Timeline. The time-travellers have a full-time tailor who takes great pains to make sure their clothing is accurate. The characters point out that their clothes do not always match their expectations.
Discussed in H. G. Wells's The Time Machine. In chapter 1, the Medical Man points out that observing the Battle of Hastings in person would attract attention: "Our ancestors had no great tolerance for anachronisms."

Live Action TV

Parodied in Time Trax, when a time traveler from the 22nd century arrives in the 1990s in something more appropriate to the 1950s. When the protagonist (who has been in the 90s for over a year now) points this out, she complains that she did the research and her wardrobe should be fine.
- A similar thing happened on Phil of the Future, when Phil (from the 22nd century) and his family find they are unable to fix their broken time machine and are stuck in 2003. Phil's parents decide to enroll him and his sister in school, and his father is charged with finding appropriate clothing so they won't stand out. Phil and Pim show up to their local high school dress in clothes stereotypically found in the 1960's. It takes them five seconds of awkward looks and laughing to realize their clothes are a little out of style.
Averted in the HBO John Adams miniseries, where the costume department took great pains to show the change in fashion from the Revolutionary War through Adams' retirement. This is most evident in the episode where Adams becomes president, wearing what is recognizable as an ancestral suit and a gray top hat.
In this instance, it's not time travel but production values. The producers of Hogan's Heroes blew their entire costume budget on those nifty reproduction Nazi outfits, so all the non-Nazi and non-main-character actors had to bring their own outfits. Thus you will see every secondary non-Nazi on the show - which takes place during World War Two - dressed in the fashions of the era of the show's production - the late 1960's.
Subverted on the News Radio episode that takes place in a futuristic spaceship. Everybody else wears "space clothes", but Jimmy comes in wearing the same suit he wears in every other show, and Space Dave mocks his ridiculous clothes. Jimmy defends the suit by saying it was the height of fashion in the late 20th century.
Played with in Star Trek Voyager. Sent back to late 20th century southern California, the voyager crew dons some outfits that "ancient history" expert Tom Paris puts together. Then they beam down and take one look at the crazy stuff people are actually wearing and Tuvok snarks that they would have fit right in wearing their uniforms.
- This is a reference to a story from the production of Star Trek IV: The Voyage Home, in which the producers sent people out dressed in Starfleet uniforms to wander around San Francisco in 1986 to get a realistic idea of how the locals would react to the time-travelling Kirk and company, only for them mostly to fit right in with the eccentric local fashions.

Webcomics

In the webcomic Get Medieval, the alien assassins chasing the main cast attempt to infiltrate medieval Earth. However, their consultant dresses them in clothes from the wrong part of the medieval era, he happens to be a linguist, without any knowledge of the actual culture that went along with the languages. Hilarity Ensues.

Western Animation

Parodied in the Futurama episode "Roswell That Ends Well"; in an attempt to fit in in 1947 New Mexico, Leela dons a poodle skirt and beehive hairdo, and Professor Farnsworth wears a zoot suit and fedora while swinging a pocketwatch on a chain. Leela also tries to fit in using Fry's 1990s slang, with similar success.
- Played straight in the episode "The Lesser of Two Evils" when they go to Past-O-Rama, which is the 30th century's take on the 20th century. Fry, attempting to steal a car he recognised, gets stopped by a worker in Renne Fair garb, says, "Sir, you can't... oh, you work here. I should've known from that ridiculous get up."
In the sequel to the movie Lilo and Stitch and in the subsequent TV show, the two stranded alien scientists, Jumba and Pleakley, don't quite get "earth culture" and in once scene attend a beach party in 19th century striped one-piece swimming suits.
- Seeing as Jumba is rather large, and mostly dark gray, and Pleakley apparently thinks he's a woman, not to mention green skinned and stick thin, the use of the whole body bathing suits might be more for camo than because they just don't get it... Of course, Pleakley has been fed a lot of misinformation about E-Arth...

Real Life

Pants/jeans and a T-shirt have been fairly common around the 20th century. Of course, what's on the Tee and the color and style of the pants definitely changed (although some styles have been more or less consistently regarded as neutral). Though there are various differences (crotch rivet, red tab, cardboard/leather patch, pocket stitching, etc.) accumulated over the years, the general design of a pair of Levi's Shrink-to-Fit 501s has been roughly the same since at least the 1920s and perhaps as far back as the 1870s (the company history is murky and often embellished with poorly authenticated statements). If you go for overalls you can probably go even further back. The key distinction is that the social acceptability of such garments is far more recent. Modern jeans didn't become popular outside of the American West until the 1930s and didn't see widespread popularity as casual, rather than work, clothes or "Western wear" until the 1950s when they were still seen as unacceptable in many situations and often banned in some places due to their association with rebellious youth.
In some areas ancient roman gladiator-style sandals are quite popular, this may be a case of "Every few thousand years, fashions come around" rather than "fashions never change".
A suit and tie have been proper formal wear for over 100 years, albeit with variations in color, fabric and things like lapel style.
- Neckties have been around for almost four hundred years.
Pirate boots, corsets, and multiple 80s-style pants are sometimes worn to school as if nothing's amiss.
The mullet. As a comedian once said, they've been 'riding it out since the 70's'.
- Longer than that, much longer than that.
Hasidic Jews wear what was the height of fashion in 18th century Eastern Europe ("Ashkenazi" Jews), even if they from the Sephardic Stream/descended (who never wore those clothes in "their" countries). Many will even dress that way in places or seasons in which the climate is far too warm for such clothing, which can result in them becoming terribly sweaty.
In various parts of the world, traditional clothing has been around for quite some time (although it varies whether it's everyday wear or for formal occasions only).
Many Anabaptist groups (such as the Amish, the Hutterites, and the Mennonites) play this trope straight to this day.
Modern Roman Catholic clerical vestments are directly descended from the secular formalwear of Ancient Rome. They have evolved, but at a tiny fraction of the pace of the surrounding culture.
A photograph achieved notoriety online in late 2010 that appears to be a reversal of this trope: A black-and-white photo from 1940 was commonly assumed to be doctored or evidence of time travel because of the presence of a man in a modern-looking sweater and sunglasses.
A white Oxford shirt has remained a staple of menswear for the entire 20th century and extends back even further as a sport shirt. Just remember that the button-down collar wasn't developed until 1896.
And while of course fashions do change, the iconic style associated with a particular era is often what the very fashionable were wearing, or the most extreme example of the current style. The ordinary person in the street may have been wearing an outfit with only a nod to the current fashion, or clothes that they had owned for years which didn't particularly reflect what was available to buy new.
Kurt Andersen writes in Vanity Fair magazine that modern fashions have barely changed since the early 1990s.