Needed a script to calcualte between two dates, took me a couple of hours to get this little script to work how I needed it to, feel free to amend it as you wish.

function days_between(date1, date2, daysbetween) {
var DateFromval = date1;
var DateFromval2 = date2;
var LengthLeaveval = daysbetween;

//split the dates while they are still in string format
var date1 = (document.getElementById(DateFromval).value);
var mySplitResult1 = date1.split(“/”);
var date1day = (mySplitResult1[0]);
var date1Month = (mySplitResult1[1]);
var date1Yr = (mySplitResult1[2]);

var date2 = (document.getElementById(DateFromval2).value);
var mySplitResult2 = date2.split(“/”);
var date2day = (mySplitResult2[0]);
var date2Month = (mySplitResult2[1]);
var date2Yr = (mySplitResult2[2]);

//rebuild the date putting into american date format and then convert to a date format
var Date1Date = new Date( date1Month + “/” + date1day + “/” + date1Yr);
var Date2Date = new Date(date2Month + “/” + date2day + “/” + date2Yr);
 
// The number of milliseconds in one day
var ONE_DAY = 1000 * 60 * 60 * 24;
//Calculate difference btw the two dates, and convert to days
daysLeave = Math.ceil((Date2Date.getTime()-Date1Date.getTime())/(ONE_DAY));
document.getElementById(LengthLeaveval).value = daysLeave;
}

Feel free to leave comments, if you feel you can improve on the above, paste your amended version for others to share.