Data validation - limit based on previous choice

Hi everybody,

I am applying data validation to cells and would like to limit the range based on a value entered in the preceding cell.

In the attached, in column A you can select any month in the list in column H.

In column B I would like to restrict the list to months after that chosen in A.

I have been trying to use OFFSET and MATCH but haven't found anything that works yet, does anybody have any advice?

I'm also not sure how to get round the problem of someone choosing column B first...

Thanks
Rich