I think Trapped in Time, Charlotte Sometimes, and Back to the Future combined are the reason I'm obsessed with time-travel stories. I was so young and impressionable then...
Audrey (Andy, for short) and her brother Nathan are a couple of New Yorkers picnicing in Prospect Park when they find an old pocket watch. When they try to wind it up again, it's hands begin to spin - backwards! They find themselves in the 1700s, and befriend a young Hessian soldier, Franz (they were hired by the British crown to work as mercenaries during the Revolution). They proceed to get Franz into some fairly serious trouble, and have to help him.
I remember reading this as a child several times. A few things that stand out to me as an adult: the children's shopping bag, full of 7Up cans and plastic sandwich bags, are merely interesting to Frnz, and ignored by everyone else. Their odd clothing as only mentioned once in passing. I assume the author meant to focus on the adventure, but it would've made more sense if they at least got some funny looks from strangers.
Along the way, they meet Elizabeth Lake. She asks to be called simply Elizabeth, and the children call her Aunt Elizabeth, but the author always refers to her by her full name. Drove me a little nuts.
A fun adventure, and a bit educational - this is where I first learned that German soldiers fought alongside the British in the American Revolution (lest you think I take my facts from fiction, I later confirmed it via my parents' set of encyclopedias).