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 the Word forum:
https://www.msofficeforums.com/word/...-tutorial.html
In particular, look at the item titled
Date and Time Calculations In A Mailmerge. Do read the document's introductory material.