InfoPath Dev

I am familiar with Access 2007 but still new in infopath 2007. I was wondering if I have two text boxes each with a time say txtstarttime and txtendtime. I want to find the difference of txtendtime and txtstarttime in a new expression box txttimedifference. Are there ways to do this without code? Please keep it simple like I said I am new. Thanks

Here is the link:

http://alecpojidaev.wordpress.com/2008/12/30/infopath-codeless-programming-walkthrough-2/

Thanks for your help