I'm writing a novel based in the mid-1970s. Would that be considered contemporary or historical fiction?
How far back is historical fiction?
Anything that can not be mistaken for the modern day, and does not rely on modern day technology. You are talking about half a century ago. I'd call that recent historical fiction.
Often historical fiction plots rely on the absence of some modern technology or modern culture. In your case, technology like cell phones and laptops, or modern cars. even the iPod. The Internet was not a thing. Virtually all selling was in retail stores, with salespeople, or by mail order.
And modern culture is much less openly misogynistic, racist and homophobic than it was in the 70's. We still had just 3 channels for TV, and national reporters were much more likely to be independent, report the truth, and never helped to spread conspiracy theories.
But 1970's political culture was when Nixon resigned because the Republicans in the Senate were clearly going to impeach him for Watergate; an honorable but laughably quaint prospect today. And we didn't worry about terrorist attacks like 9/11, or mass shooters.
Those kinds of details (and many more) make the 70's clearly not the modern world, and clearly historical fiction.
Don't get confused by genre labels. Set in the past won't make the work 'Historical Fiction', any more than a relationship subplot within a story makes it a 'Romance'.
The distance in time is not what's important. Historic Fiction set further back in time is not 'more historic' – ad infinitum: stories about dinosaurs are not 'historic fiction' simply because they take place longer ago. How recent is splitting hairs, but also contextual.
A story's setting says more than just time and place. It's how people thought and what they believed. It's how lives were shaped by the times, what resources they had available, and the limits of their knowledge.
The 'history' should be far enough in the past that the average reader has knowledge of events the characters do not. We are experiencing their story with the benefit of hindsight. We know how this turns out. We are primed by their naiveté and dogma. This is part of historic storytelling, whether it's plot crucial or just exotic charm.
The genre label 'Historic Fiction' implies that history will play a dominant role in the story. The circumstances of that time and place will impact the plot. The common beliefs of the day will handicap character arcs. It's more than cosmetic set-dressing, it's integral to the story.
Is your story about a specific time/place in history – ie: plot and characters designed to show 'slice of life', with historically accurate incidents and typical beliefs/political views based on research of that era?
The stock narrative structure of historic fiction is (reductively) an ensemble of characters who personify various aspects of society, probably voicing the conservative and progressive beliefs of their day – heroes are on the winning side of history, antagonists cling to out-dated values. The plot catalyst is an historic event or an inevitable generational shift that will personally impact their lives (in a big or small way), allowing the various characters to seek their new normal in this changing environment. Status quo is disrupted ⇨ stuff happens ⇨ new status quo.
Works of nostalgia idealize aspects of the recent past that are now considered lost or waning, ie: the simplicity or innocence of a bygone era, speaking to a generation's childhood experiences and perceptions.
Our media repeats itself every generation (2-3 decades), resurrecting kid's merchandise and elevating it to icon. Nostalgia speaks to a specific tribe and generation, but exaggerated, spoofed, or idealized. Nostalgia isn't so concerned with actual history, rather the perception of history. It's a scrapbook of emotional associations.
Ad infinitum, nostalgia will enshrine a yesteryear that neverwas, several generations removed from the (factual) source material. Not every bachelor had a hot tub and waterbed in the 1970s, but generational memories are now exaggerated. Hot tubs and waterbeds represent a nostalgic ideal that has compounded into a trope of "The 1970s".
Every generation will get a grittier Batman, a more explode-y Star Trek, a more serious Space Opera…. These things in no way resemble their source material, they are nostalgia compounding. And regenerating (not a bad thing).
If the setting was chosen to deliberately draw contrast or parallel to current events, it's probably allegorical – ie: Arthur Miller's The Crucible is set during the Salem witch trials but intended to read as an allegory to current (1953) politics.
The protagonist may have questionably modern sensibilities aligning with the reader, while other characters embody the worst aspects of out-dated beliefs. While not necessarily anachronistic, it is in service of the story and theme.
Genres are more than tropes, they are also the structure of the story, and a bag-load of reader expectations including character arcs, plots, and theme. Labeling your story with a specific genre is a promise to the reader.
An easy way to circumvent genre labels is to compound them, ie: 'Historic Horror' is not an actual genre, but readers can assume what story elements it would combine. A horror story set in the past will still primarily follow 'Horror Genre' structure, even if the setting is iconic, ie: Pride and Prejudice and Zombies doesn't need to be explained, and is a promise to readers looking for that specific Regency era/apocalyptic survival mashup.
'Punk' is about one aesthetic, dialed up to eleven. Similar to nostalgia, but purely about the style of a time/place, rather than emotional trigger-memories from childhood.
Punk is a rococo lens that exaggerates world-building – to the point of straining credibility. To balance, characters and plots are simplified so as not to burden the narrative. Pacing is probably the only definitive structure to 'punk'.
Indiana Jones is pure popcorn (with nostalgia butter) – despite the dieselpunk nods to 1930s adventure serials the structure is a thoroughly modern action film where every character is invested in globe-hopping for treasure. The tropes are borrowed, the costumes are period, but 'historic fiction' is not the marketing label that springs to mind.
Ad infinitum: The Jetsons is rocket-punk. The Flintstones is caveman-punk. They take place in an imaginary time when everything is that one crazy thing but at the same time universally relatable. It's shallow worldbuilding, but piled high.
Any historic setting can be punked. Any sub-genre can be punked. It's about adding more sugar and food-coloring to the Kool-ade. Historic purists won't be offended because it's not pretending to be history, ie: Bridgerton is not universally hated by vintage garment researchers because they see it is Regency-punk, and they go with it.
Genre is mostly a marketing term. Use it to reach consumers that will be interested in your story. More importantly, you don't want to alienate readers with the wrong label, or promise them the wrong genre expectations.
You can always lean in to the historic, nostalgic, allegorical, punk aspects that help you tell a richer story, but don't get too hung up on the labels, unless you are trying to target that specific reader market.
If you aren't sure, it probably isn't.