Ideally, do the addition in a query or view in your database - this is
technically possible in e.g. Access, but you may not have permission to do
it.

Otherwise, you can try the following nested fields for this one:

{ QUOTE "{ SET Y { = { MERGEFIELD mydate \@YYYY }  + 1 }
}{ SET M { MERGEFIELD mydate \@MM }
}{ SET D { MERGEFIELD mydate \@DD }
}{ IF { REF M } = "02"
"{ IF { REF D } = "29" "{ SET M "03" }{ SET D "01" }" "" }" ""
}{ REF Y }-{ REF M }-{ REF D}" \@"MMM DD, YYYY" }

All the {} need to be the special field codes you can enter using ctrl-F9.
substitute your date file name for "mydate", and substitute the date format
specification you need instead of "MMM DD, YYY"

If you need a more general-purpose routine for manipulating dates using Word
fields (and even if you don't it's probably worth checking out), I suggest
you head for macropod's excellent resource at

http://www.wopr.com/cgi-bin/w3t/showflat.pl?Cat=&Board=wrd&Number=249902