DateDiff(interval, date1, date2[, firstdayofweek[, firstweekofyear]]) returns the interval between two specified dates : DateDiff « Date Functions « VBA / Excel / Access / Word
DateDiff(interval, date1, date2[, firstdayofweek[, firstweekofyear]]) returns the interval between two specified dates
Constant Value Year Starts with Week
vbUseSystem 0 Use the system setting.
vbFirstJan1 1 The week in which January 1 falls (the default setting).
vbFirstFourDays 2 The first week with a minimum of four days in the year.
vbFirstFullWeek 3 The first full week (7 days) of the year.