I have made a start on this.

In cell A8 you enter the date, in C8 you enter the frequency in weeks and in F8 you get the next date. For this I have this formula.
=IF(ISBLANK(A8),"",WORKDAY((A8+C8*7)-1,1,))

Now and again a number per calendar month might have to be entered. Is there a way to have this also in C8? If needed I can make sure only "1M" "2M" "3M" etc is entered.

This is the formula I have started on for the per calendar month, but it would be nice to not have another column just for the occasional per calendar month.

=DATE(YEAR(A8),MONTH(A8)+D8,DAY(A8))

Any help would be great!

Thanks