Date comparison in javascript

function validDeparturDate(date_arrival,date_departur){

var str1 = date_arrival;
var str2 = date_departur;
if(trim(str1)!="" && trim(str2)!="")
{
var yr1 = parseInt(str1.substring(0,4),10);
var mon1 = parseInt(str1.substring(5,7),10);
var dt1 = parseInt(str1.substring(8,10),10);
var yr2 = parseInt(str2.substring(0,4),10);
var mon2 = parseInt(str2.substring(5,7),10);
var dt2 = parseInt(str2.substring(8,10),10);
var date_arrival1 = new Date(yr1, mon1, dt1);
var date_departur2 = new Date(yr2, mon2, dt2);

if(date_departur2 < date_arrival1)
{
return false;
}
}
return true;
}

if(trim(g.date_of_departure.value)=="")
{
alert("");
g.date_of_departure.focus();
return false;
}
var date1 = g.date_of_arrival.value;
var date2 = g.date_of_departure.value;
if(trim(g.date_of_departure.value)!="" && !validDeparturDate(date1,date2))
{
alert("Date of departure should be grater then date of arrival!");
g.date_of_departure.focus();
return false;
}