Excluding a row from being filtered

Hi Folks,

Before I kick off I must say what a top notch solution this has proved to be, congrats to the dev guys :)

My question is this I have a column of data that contains dates and I'm using the date range filter to show/hide the matching dates which is fine. The problem that I have is that on some rows the date field isn't populated (well it is but with the text of NA). When a row contains NA in the date field I always want these records to be visible ie. Regardless of what date range a user selects the records where the date isn't applicable should always show in the datatable.

I've had a hunt around but can't seem to find a solution....

Could anybody help please?

Thanks in advance.
Craig