The field coding for this is rather more complex than just taking a DATE field's value and adding 14. 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 in the 'Sticky' thread at the top of this forum:
Microsoft Word Date Calculation Tutorial
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.