History Student whose sole purpose in life is nitpicking out pointless details and writing meaningless posts (or as my professors call them, “response papers”) about them here!
Generally speaking, exact dates aren’t too important. Unless the exact chronology of your story exists in contradiction to major historical events (meeting Lincoln in Connecticut on the day of the Gettysburg Address, for example), there’s really no need to count hours. I wouldn’t worry too much about it, unless you’re setting your story around major historical events, in which case, you should probably put some effort into getting those right.
In any case, you’ll never manage perfect historical accuracy, even if you put in years of research, especially if you need to bend the records for the sake of your story. Not only are there going to be blank spots in the historical record, there are also ongoing academic disputes regarding certain points of contention. The respective strengths of armies in major battles, for example: the defeated side tend to inflate the size of the winning force to make their defeat more excusable. The victors tend to inflate the size of the losing army to make their own victory more impressive. You even see this all the way up to the Second World War, and to some extent, in less scrupulous media outlets today. The historical record isn’t an objective thing, rather a record of the past as recorded by subjective witnesses, and studied by subjective experts with their own biases and prejudices. While there’s a consensus on a vast majority of major historical events and trends, there are still the ragged edges, and what might seem fine to one historian, might stand out as a glaring error to others.
What’s more important is what some media critics call “historicity”, or what can be described as “look and feel”. This is the part of history that often gets ignored, because it doesn’t show up on standardised tests, but it’s also what you should be concentrating on if you’re looking for authenticity. It’s a hard thing to quantify, but I suppose the best way to sum it up is to divide it into three questions: “how did people live?”, “how did people think?” and “how did people exist in relation to each other?”
The first question is probably the easiest, because you’re dealing mostly with material affairs: what technologies are available, what’s been or hasn’t been built, what’s in fashion, what is considered outdated, or too gauche. If you’re setting your story in a specific time, you’d want to study how people worked, what people ate, how they worked, what they wore (there’s a special hell for people who assume any style of clothing will do for a specific historical setting just because it "looks old). There’s plenty of examples of all this stuff widely available though, and generally speaking, it’s pretty easy to fill in any blanks in your knowledge with a simple google search.
Directly related to this is the second question, and this is more important to you as a writer than as a historian: The past truly is a different country, with different mores, manners, and personal priorities. It’s important that you take that into account, but it’s also rather difficult to pull off: while some of the details (like period dialects) can be fudged for the sake of comprehensibility, you’ll want the general patterns of behaviour and manners down: things like the ways people sign their letters, or greet each other on the street help establish “this is the past” to a casual reader, and “the author did their research” to a more studied (or anal-retentive) one. In any case, you’ll definitely want to do some research on how people any particular time thought of themselves, the world, and the people around them, which leads onto the final question: how individuals see themselves and others within their society.
People don’t exist in a vacuum. Perhaps the most important part of a person’s thought is how they view not just other individuals, but the broad categories of people outside their personal sphere which they have no personal experience or empathy with: who would a given person consider acceptable to associate with? Who would they find below them, or above them, and how would they treat those people? Find out the different ways which people of different genders, races, and social stations interacted with each other. Likewise whether you plan on actually implementing historically accurate attitudes of racism, sexism, homophobia, and classism is entirely up to you, but keep in mind that a lot of the ways which people interact with each other are informed by these prejudices on an unconscious level. If you want your historical fiction to fit together, you’re going to need to take that into account.
Just some thoughts, feel free to take them or leave them.
As a huge naval history nerd, I would go ballistic if someone messed up by say, putting hermaphrodite cruisers or galleons with overbuilt castles in the 1850s. It’s that sort of error that really throws off the history buff.
Generally, it helps to determine what the big things you need to get right are, and what things will get a pass simply out of obscurity or the simple expedient of “it’s still plausible, there are rare historical examples.”
I’ll shut up now.