Need a formula to calculate holidays from start dates to the current date!

Basically, I've a holiday tracker for learners. I have their names in columns a & b, their start dates, then in the top row going across I got the week commencing etc. I need to create a formula that works out how many months it has been since a learner has been here, times that by 1.5 (they get 1.5 days a month holiday) then - (minus) the days they have already taken. is there ANY WAY of doing this? My file is attached.
Thanks in advance.