Basically it's just =(date2-date1)/7 where date2 and date1 are cell
references for your two dates.
=ROUND((date2-date1)/7,0) if you want to round to the nearest whole week
=CEILING((date2-date1)/7,1) if you want to always round up to the nearest
whole week
=FLOOR((date2-date1)/7,1) if you want to always round down to the nearest
whole week
Hope this helps.