Really good question. I didn't know the answer off the top of my head.
Anyway, try something like this:
var start_date = document.getElementById('startdatectrl').valueAsDate;
var end_date = document.getElementById('enddatectrl').valueAsDate;
$.ajax({
type: "POST",
url: "php/getyourdates.php",
data: { startdate: start_date, enddate: end_date }
})
.done(function (msg) {
});
Note that you will need to give your date controls id's.
Also, I recommend using POST because it is more secure and flexible (in terms of the data it can send).