In ASP, you can use DateDiff to calculate the difference between two times, but it is not available in javascript and requires a handwritten function.
js can actually directly Date1-Date2, and it can be converted into milliseconds to calculate the time difference.
// Calculate two dates The number of days between
function DateDiff(sDate1, sDate2){ //sDate1 and sDate2 are in 2002-12-18 format
var aDate, oDate1, oDate2, iDays
aDate = sDate1.split("-")
oDate1 = new Date(aDate[1] '-' aDate[2] '-' aDate[0]) //Convert to 12-18-2002 format
aDate = sDate2.split("-")
oDate2 = new Date(aDate[1] '-' aDate[2] '-' aDate[0])
iDays = parseInt(Math.abs(oDate1 - oDate2) / 1000 / 60 / 60 /24 ) //Convert the difference in milliseconds to days
return iDays
}