difference between two dates

hey everyone!

I am trying to calculate the difference between two dates which are inputted into textboxes.

          
Start Date: <input name="start_date" type="text" id="start_date" onClick="GetDate(start_date);" onChange="duration();">

End Date: <input name="end_date" type="text" id="end_date" onClick="GetDate(end_date);" onChange="duration();">

HTML:

GetDate(); loads an in-line calendar while duration() is supposed to work out the difference and write it to a third textbox.

This is the Javascript I am using to try and work it out:

function days_between(date1, date2) {
    var ONE_DAY = 1000 * 60 * 60 * 24
    var date1_ms = date1.getTime()
    var date2_ms = date2.getTime()
    var difference_ms = Math.abs(date1_ms - date2_ms)
    return Math.round(difference_ms/ONE_DAY)
}

function duration() {
var from_date = document.getElementById('start_date').value
var to_date = document.getElementById('end_date').value
var duration = days_between(from_date, to_date)
document.getElementById('duration').value = duration
}

Code (markup):

but the duration isn't updating at all!

Here is another function I tried which doesnt seem to work:

var day1, day2;
var month1, month2;
var year1, year2;

value1 = form1.start_date.value;
value2 = form1.end_date.value;

day1 = value1.substring (0, value1.indexOf ("/"));
month1 = value1.substring (value1.indexOf ("/")+1, value1.lastIndexOf ("/"));
year1 = value1.substring (value1.lastIndexOf ("/")+1, value1.length);

day2 = value2.substring (0, value2.indexOf ("/"));
month2 = value2.substring (value2.indexOf ("/")+1, value2.lastIndexOf ("/"));
year2 = value2.substring (value2.lastIndexOf ("/")+1, value2.length);

date1 = year1+"/"+month1+"/"+day1;
date2 = year2+"/"+month2+"/"+day2;

firstDate = Date.parse(date1)
secondDate= Date.parse(date2)

msPerDay = 24 * 60 * 60 * 1000
dbd = Math.round((secondDate.valueOf()-firstDate.valueOf())/ msPerDay) + 1;
document.getElementById('duration').value = dbd

Code (markup):

 

Yes it works