Two ways to do this with IP 2003, neither real nice. The best method is just
to use a bit of code.
You can use the datediff function in VBScript.  There are several examples
at http://www.infopathdev.com Just search on Datediff.

The other, low tech method is break the date down into its components,
increment the day and then put it back together, but this can lead to bad
dates say when it is the 31st unless you build some other logic/rules to know
when to change the month. This can be done with substring functions.