Thanks for the replies by the way, I really appreciate the help.

I have an SQL database I'm pulling the date fields from and here is what happens. Unfortunately I can't modify the date in the database at all, so I have to do it all on the letter.

Paid_to_date = 16/08/2011.

Invoice from <delay by 1 day> to <delay by 1 month>

The above merge fields pull through the following information.

Invoice from 17/08/2011 to 16/09/2011.

However without changing anything, if I create the letter from a file with a paid to date anywhere in February, it does the following:

Paid_to_date = 14/02/2011

Invoice from 12/02/2011 to 14/03/2011.

If I set the delay to 3, instead of 1. February then works fine. Maybe I typed something wrong. When I've a chance I'll double check it. But if the above field is correct, my letter should be correct.