Grey Out / Disable Cells In A Row Based On Frequency Value

Essentially, I'm creating a spreadsheet which contains a list of staff in rows, and the months of the year as columns. It is to record certain company checks which must be done (e.g. driving licence check). When a check is complete, the date it was done is entered under the corresponding month. Certain checks (e.g. driving license) only need to be done every 6 months. What I'm trying to do is find a formula/vba to look at each row, determine where (which month) the date was entered in, and grey out the 5 months in front, meaning the next check is required in 6 months time.

Ideally, the frequency of this check is dynamic, and can reference a cell (e.g. might change to every 3 months etc).