why dont you use datepicker  like ?

var olddate =  new date("1/1/1990);
var currentdate =  new Date();
$( ".selector" ).datepicker({ minDate: olddate, maxDate:currentdate  });

reference http://jqueryui.com/demos/datepicker/

but if you anyways want to validate try following

function isValidDate(startDate,endDate,datetocheck) {
   var	sd = Date.parse(startDate);
	var ed = Date.parse(endDate);
	var dc = Date.parse(datetocheck);
	if((dc <= ed && dc >= sd)) {
		return true;
	}
	return false;
}