To see how to do this and just about everything else you might want to do with dates in Word, check out my
Microsoft Word Date Calculation Tutorial, at:
http://www.msofficeforums.com/word/38719-microsoft-word-date-calculation-tutorial.html
or:
http://www.gmayor.com/downloads.htm#Third_party
In particular, look at the item titled Calculate a day, date, month and year, using n days delay. Do read the document's introductory material, as you'll also need to change the output coding slightly. Other than that, you can simply copy & paste the
already-working field from the tutorial into your own document, then change the 'Delay' setting from 14 to 30.
Note: You can't reliably use Charles' field coding for anything more than 28 days.