What if you were to just toss a cup of scenes onto your pages? Each one was a glowingly written pearl, error free, and just plain awesome. They all involved the same characters. They all could have taken place today in hometown USA. Would you read this book?
I am such a linear creature, reading this random collection of scenes would likely drive me nuts. I am always in search of a progression of events, a character arch of some sort, a resolution to go with the beginning. This is why I don't care for a book of short stories. I actually have to gear myself up for such a book - prepare myself for the fact that the book is full of separate stories and not chapters. I've been taken by surprise a time or two and was really frustrated - didn't pay close enough attention to the title when I picked the book up.
But I digress - Back to our cup full of lovely pearls in need of a string. The string of your story is very important. One scene needs to lead into the next one, and the next one, and on and on. You can't have Mary kissing Billy before she meets him. You can't have Billy showing off his baby boy before he ever knows Mary has moved in next door.
Now let's really make this interesting and mix time travel in. Does that fix our issue with the cup of pearls? It doesn't. It can't. Time travel can be fun to mess with; you get to visit all sorts of interesting places in time, or visit different spots in someone's life, as might be the case in this sample.
Even with time travel in full play, we still need a string for our pearls. The best way to do this is to pick at least one character in your story. I'll flip a coin and...Billy is my main. Regardless of how many loops through time the story takes, the events that happen around Billy happen in order. Billy needs to do this, which means that he needs to go there, which lets him meet Mary, which causes him to do something else so he can go back and kiss Mary, which isn't enough so he needs to go back to the beginning and give his assignment to someone else, so he can go back and win Mary's hand. But then for some reason he needs to go back even further to set some detail right, and... Well you get the idea.
It doesn't matter how many loops and trips through time, forward or backward, there still needs to be something linear, something we can follow from a beginning to an end, through a character arch. Look at Billy. No matter how he traveled through his story, he went from an irreverent time traveler to looking to commit to a relationship, to being a proud daddy. That progression, no matter how glowing the scenes are, is not possible without a string.
Do you tell your story in order? Now, I'm not talking about the opening scene that is essentially the ending, and then the story backs up and gets us there. That's different. Besides, it still has a string. It's just that the clasp is on the other end of the necklace. haha