Date Comparison Validation

I have 2 textbox for Date input: Start Date and End Date I have javascript onsubmit="return fieldCheck()" to check all the required fields are not empty.

My problem is that i want to throw below javascript to check that End Date cannot be earlier than the Start Date. It works if both dates are in the same month, and if they are different month or day, then it's not working properly

d1_str = document.form.txtEndDate.value;
d2_str = document.form.txtStartDate.value;

d1 = new Date(d1_str.split('/')[2],d1_str.split('/')[1],d1_str.split('/')[0]);
d2 = new Date(d2_str.split('/')[2],d2_str.split('/')[1],d2_str.split('/')[0]);

if (d1.getTime()<d2.getTime()) {
emptyfields += "
End Date can't be earlier than Start Date" ; //this is the error msg
}