The VB DateDiff function requires the earlier date to be placed first and
the later date second... you have them reversed. Try this line in your
function instead...

session_num = DateDiff("ww", start_date, today_date)

--
Rick (MVP - Excel)


"Mijosuva" wrote in message
...
Dear Expert

I try to use DateDiff but it doesn't work, when I try to use this simple
function excel say #NAME?

This is what I have

Function session_num(start_date As Date, today_date As Date) As Long


'Determine the # of weeks between today date and Start Date

session_num = DateDiff("ww", today_date, start_date)


End Function