Timelines of tv shows and movie franchises are extremely important when writing fanfiction, especially if there are several years and backstories involved. For example, Twilight. A majority of the characters are vampires and have lived for many, many years. Things have happened in those years, which might have been a big part of the story or simply slipped in. It's important to get all of these details right if they come up in your book. You cant say the altercation with the Volturi happened in June because it specifically happens in winter, when the snow sticks.
It's easy to find a timeline online, so just look one up and keep it open while you're writing so you get all the facts right. I don't know about you, but I find timelines to be extremely helpful while writing.
Take Teen Wolf for another example. That show barely ever gives you anything concrete with dates and time, so it's nice for someone else to figure out an estimation of them.
It's important to stay true to the established timeline (don't even get me started on McGonagall being in Crimes of Grindelwald), unless it's an AU, because that's the whole point of what you're writing about. The time in between the events is important, sometimes even the date of the event is important. So just look it up if you aren't sure of something.
If anyone would like for me to post actual timelines on here, let me know!