Hi @Lauren Griessel, thank you for taking the time to reply to my question.
I have added this formula in and changed the Purchase date to Delivery Data
=if(Months(CurrentDate())<Months(Delivery Date),"",Months(CurrentDate())-Months(Delivery Date)
However its come up with an error

I think but am not sure this could be due to the delivery data format? example below

Do you agree? and if so do you know what format it should be?
Appreciate any help you can give me:)