Jetset is like a really light version of Dopplr, which I love, but don't want to use that much, ya know? It's really just one PHP function which is called--you guessed it--parse_locations. It accepts an associative array of locations and dates (dates are keys, locations are values), sorts them, and then returns a new array. That new array has a few keys of its own:
current: Where you are right now.previous: The last place you were.next: Where you're going.past: An array of all the places you've been before (not including what's inprevious).future: An array of all the places you're going to be (not including what's innext).
Each returned item is its own array, with two keys: date is the location's key in the original array, and place is whatever you passed in originally. I'm using city names, but this can be whatever you'd like. Maybe you want some microformat-friendly fields. Go for it.
I like storing all of these in a Yaml file, parsing that, and passing it to the function, because then I can stay as far away from PHP as possible. Since the array is sorted later, I can be sloppy and dump dates in as I think of them.
That's all. See a live copy of this at http://johnholdun.com/jetset.