How to add 3 months to existing date mergefield in Word

This will not be simple, but it can be done.

Download and open Paul Edstein's DateCalc tutorial.

Microsoft Word Date Calculation Tutorial

Be sure to read the introductory material. He has complex fields for different situations that you can copy and paste into your documents. Here you would use your initial mergefield as the starting point.

Look at "Calculate a day, date, month and year, using n months delay." You can link directly to that field using the Table of Contents.

This has been my go-to source for many years when I need such a field.

Do not try to type or create the following; instead copy the real Word field from Paul's tutorial into your document and edit it there.

{QUOTE
{SET Delay 9}
{SET mm{=MOD(ABS({DATE \@ M}+Delay+11),12)+1}}
{SET yy{=INT({DATE \@ yyyy}+(Delay+{DATE \@ M}-1)/12)}}
{SET dd{=IF(({DATE \@ d}>28)*(mm=2)=1,28+((MOD(yy,4)=0)+(MOD(yy,400)=0)-(MOD(yy,100)=0)),IF((mm=4)+(mm=6)+(mm=9)+(mm=11)+({DATE \@ d}>30)>1,30,{DATE \@ d}))}}
"{dd}-{mm}-{yy}" \@ "dddd, d MMMM yyyy"}

You would change the delay from 9 months to 3 months and use your mergefield instead of the DATE field everywhere the DATE field is shown above.

See also: How to type Fields in Microsoft Word and How to Toggle the Display of Field Codes in Word (my page).