This is not that easy to do during the mailmerge operation.   The easiest
thing would be to move your data source into and Excel spreadsheet and use
Excel's ability to handle the data arithmetic and then use the Excel
spreadsheet as the datasource.

As Doug says, this is not easy, but it can be done. Download the sample
document linked from
http://www.gmayor.com/insert_a_date_other_than_today.htm and modify the
field code changing DATE to Mergefield Datefieldname and the delay from 301
to 10.

The result will be something like

{QUOTE{SET Delay 10}{SET a{=INT((14-{MERGEFIELD FIELDNAME \@ M})/12)}}{SET
b{={MERGEFIELD FIELDNAME \@ yyyy}+4800-a}}{SET c{={MERGEFIELD FIELDNAME \@
M}+12*a-3}}{SET d{MERGEFIELD FIELDNAME \@ d}}{SET
jd{=d+INT((153*c+2)/5)+365*b+INT(b/4)-INT(b/100)+INT(b/400)-32045+Delay}}{SET
e{=INT((4*(jd+32044)+3)/146097)}}{SET f{=jd+32044-INT(146097*e/4)}}{SET
g{=INT((4*f+3)/1461)}}{SET h{=f-INT(1461*g/4)}}{SET
i{=INT((5*h+2)/153)}}{SET dd{=h-INT((153*i+2)/5)+1}}{SET
mm{=i+3-12*INT(i/10)}}{SET yy{=100*e+g-4800+INT(i/10)}}{=dd*10^6+mm*10^4+yy
\# "00'-'00'-'0000"} \@ "d MMMM yyyy"}

Credit to fellow MVP 'Macropod' for the work put into this.