week numbering

Hi all.
I'm running a spreadsheet that manages lessons in a certain course.
I have a cell that contains the date of the first day in the course, which changes, of course, from one course to another. Note that the course can begin in any day and not necessarily on the beginning of the week.

In the list of the lessons, each lesson has its date.
I'm looking for a formula that would give me the week number of the lesson, from the beginning of the course (column G in the attached file).
For example, if the course had started on Tuesday, September 18th, a lesson on Sunday, September 23rd would be on the second week of the course.

I cannot use the function "weeknum" because it is a part of the Analysis ToolPak add-in which I cannot install.

Thanks a lot.