DateDiff - function of language VBScript

Returns the number of intervals between two dates.
DateDiff(interval As String, date1 As Date, date2 As Date, [firstdayofweek As Integer], [firstweekofyear As Integer]) As Long
n = DateDiff(interval, date1, date2[, firstdayofweek[, firstweekofyear]])
interval(String) Text string that is the interval you want to use to calculate the differences between date1 and date2
yyyy - year
q - Quarter
m - month
y - day of year
d - day
w - day of the week
ww - week of year
h - hour
n - minute
s - second
date1(Date) First date in the calculation
date2(Date) Second date in the calculation
firstdayofweek[optional] (Integer) Specifies the day of the week. If not set, then Sunday is assumed (values see VBScript Date and Time constants).
firstweekofyear[optional] (Integer) Specifies the first week of year. If not set, then the first week is assumed to be the week in which 1st January occurs (values see VBScript Date and Time constants).

You can use the function to determine how many specified time intervals exist between two dates. For example, you might use function to calculate the number of days between two dates, or the number of weeks between today and the end of the year.

To calculate the number of days between date1 and date2, you can use either day of year ("y") or day ("d"). If interval is day of the week ("w"), then function returns the number of weeks between the two dates. If date1 falls on a Monday, then function counts the number of Mondays until date2. It counts date2 but not date1. If interval is Week ("ww"), however, then function returns the number of calendar weeks between the two dates. Function counts the number of Sundays between date1 and date2. Function counts date2 if it falls on a Sunday; but it doesn't count date1, even if it does fall on a Sunday.

If date1 refers to a later point in time than date2, then the function returns a negative number. The firstdayofweek argument affects calculations that use the "w" and "ww" interval symbols.

If date1 or date2 is a date literal, then the specified year becomes a permanent part of that date. However, if date1 or date2 is enclosed in quotation marks (" ") and you omit the year, then the current year is inserted in your code each time the date1 or date2 expression is evaluated. This makes it possible to write script that can be used in different years.

When comparing December 31 to January 1 of the immediately succeeding year, function for year ("yyyy") returns 1 even if only a day has elapsed.

Detects the number of days between a given date tDate and today
s = "Days from today: " & DateDiff("d", Now, theDate)
PROMOTIC 8.3.24 SCADA system documentation - MICROSYS, spol. s r.o.

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