"My flashbacks are dated so no problem there."
Although don't forget that an awful lot of readers actually don't read headings, chapter title, datelines and stuff. I know, because, try as I will, I don't - my eye shoots straight over them to get on with the story. I've trained myself deliberately to read them, for the purposes of teaching and doing appraisals, but it still doesn't come naturally.
The thing is, even if readers do read dates or whatever, we have to have remembered the previous date, to know how it relates to this one and then understand the significance of how much time has passed, or that we've shot backwards, or whatever. Essentially, if a reader has to stop and work it out - let alone flip back in the book to remind themselves of other dates and places - then the writer has failed. A date on its own means nothing - unless maybe it's a date that is famous in history. Even then, its significance for the story can't flower properly if the reader doesn't have another anchor from before, to relate it to. Places work better, because they log more easily in the head for most of us, though if it's all obscure villages, not "London" followed by "Paris", it may not work all that well.
My rule of thumb is that you need two
hand-holds for the reader in the opening line or three after every abrupt switch of time or place. One might be a dateline or some such, but the other hand-hold had better be something which operates on us intuitively, by being embedded in the actual narrative: voice, names, material facts, perhaps a switch of tense.
This blog-post on different ways of getting from one scene to the next also applies to moving to and fro in time and place:
And this is about different ways of handling backstory and bits of the past: