I'm trying to remember this book I read like a decade or so ago, I don't know if that's when it actually came out. I think it was a young adult book, and it was a time travel romance, from the POV of a boy who met a time traveller and eventually fell in love with and married her. She showed up at various points in his life, not in the same order for her. And the POV character was the only one to ever remember that time had changed somehow, which is why she interacted with him at first? An early scene of him as a kid was specifically about his bike changing color from blue to red or visa versa. They end up married at some point and he has to remind her about their history together because she is always off in these time travelling adventures between visiting him.
It's killing me that I can't remember what the book is called! If anyone knows what it might be please let me know I'm really curious to reread it.
Edit: I JUST REMEMBERED they had a codeword at some point to say whether time had changed or not, it was "stingray" or something similar