From http://www.w3schools.com (Copyright Refsnes Data)

VBScript DateDiff Function

The DateDiff function returns the number of intervals between two dates.

Syntax

DateDiff(interval,date1,date2[,firstdayofweek[,firstweekofyear]])

Parameter	Description
interval	Required. The interval you want to use to calculate the differences between date1 and date2 Can take the following values: yyyy - Year q - Quarter m - Month y - Day of year d - Day w - Weekday ww - Week of year h - Hour n - Minute s - Second
date1,date2	Required. Date expressions. Two dates you want to use in the calculation
firstdayofweek	Optional. Specifies the day of the week. Can take the following values: 0 = vbUseSystemDayOfWeek - Use National Language Support (NLS) API setting 1 = vbSunday - Sunday (default) 2 = vbMonday - Monday 3 = vbTuesday - Tuesday 4 = vbWednesday - Wednesday 5 = vbThursday - Thursday 6 = vbFriday - Friday 7 = vbSaturday - Saturday
firstweekofyear	Optional. Specifies the first week of the year. Can take the following values: 0 = vbUseSystem - Use National Language Support (NLS) API setting 1 = vbFirstJan1 - Start with the week in which January 1 occurs (default) 2 = vbFirstFourDays - Start with the week that has at least four days in the new year 3 = vbFirstFullWeek - Start with the first full week of the new year

Example 1

document.write(Date & " ")
document.write(DateDiff("m",Date,"12/31/2002") & " ")
document.write(DateDiff("d",Date,"12/31/2002") & " ")
document.write(DateDiff("n",Date,"12/31/2002"))

Output:

1/14/2002
11
351
505440

Example 2

document.write(Date & " ")
'Note that in the code below
'is date1>date2
document.write(DateDiff("d","12/31/2002",Date))

Output:

1/14/2002
-351

Example 3

'How many weeks (start on Monday),
'are left between the current date and 10/10/2002
document.write(Date & " ")
document.write(DateDiff("w",Date,"10/10/2002",vbMonday))

Output:

1/14/2002
38

Complete VBScript Reference