Keeping a story’s timeline accurate is important for many reasons to include controlling flashbacks.
I have read flashbacks that were to be one year ago, but were several years based on events. Or a twenty-year-old who five years ago was eighteen. The seasons seem to confuse writers. Going back one season from spring is winter, not summer. Also, flashing back to fall can be confusing when the present is also fall. What fall is what?
I think the biggest issue is too many flashbacks. The reader can get lost between the present and the past and lose the story in the confusion. Flashbacks should be like seasoning. Too much and there is no taste. I try to limit my flashbacks like I limit my adjectives and adverbs.
Flashbacks are good when they present a backstory that reveals a climax in current time.
I think flashbacks should provide the reason “why” actions are done and give more depth to the characters. They are not to provide another story that has no conclusion and belongs in a separate work. Also, flashbacks should be limited to only the basic information. The writer does not need to include everything.
Some writers try to avoid flashbacks by using prologues. But do readers read them? Another way writers avoid flashbacks is to start at the beginning. But that could be too far in the past and include too much telling to bring the story to the present.
I recently listened to a podcast where the speaker did not like flashbacks. I think stories can include flashbacks, just treat them like any modifier. They enhance the story and can be useful when not overused.
So, use flashbacks, but sparingly. Think of looking behind yourself while walking. The glance back can’t be too long or you’ll walk into something.