int = WeekOf(nDate)
int = WeekNumberOf(nDate)
Both functions return the week number corresponding to nDate. The return values range from 1 to 53.
This function uses the ISO week numbering scheme : week 1 of a given year is the first week containing a Thursday. Another way of thinking about it is that if a week contains a Thursday in the new year, then that week has at most 3 days in the previous year, and at least 4 days in the new year. So it has the majority of its days in the new year and is the first week of that new year.