That's because your dat1 and dat2 variables are just strings.
You should parse them to get a Date object, for that format I always use the following function:
// parse a date in yyyy-mm-dd format
function parseDate(input) {
var parts = input.match(/(\d+)/g);
// new Date(year, month [, date [, hours[, minutes[, seconds[, ms]]]]])
return new Date(parts[0], parts[1]-1, parts[2]); // months are 0-based
}
I use this function because the Date.parse(string) (or new Date(string)) method is implementation dependent, and the yyyy-MM-dd format will work on modern browser but not on IE, so I prefer doing it manually.