Data Validation / Formatting Help

Hi All

I have created a drop down menu using data validation (Cell F3) but I want the format of Cell C7 to change to "Months" when I change the dropdown data in the data validation cell from "Yearly" to "Monthly". I am not sure if it is possible to enter an "If" formula into the Custom Number Format box (I can't seem to get it to work correctly)?

Any help/alternative suggestions on this would be great - many thanks!

Regards

SilverBullet