I am looking to subtract one date from the other and based on the result have the field (multiple choice) value change. For example (field1-field2) > 2days = "yes"
datediff function in InfoPath doesn't exist.
what I would do is
https://alecpojidaev.wordpress.com/2008/12/30/infopath-codeless-programming-walkthrough-2/
haven't played with this myself but let me know if you do and it works