InfoPath Dev

I have string data type field named startDate with text in the format of YYYY/MM/DD, I need to convert the text to a format that I can use to set a data picker named ppDate without getting a validation error Is there a way to accomplish this using substring-before, substring-after and concat ?

solved my issue with http://blogs.msdn.com/b/infopath/archive/2007/02/21/calculating-elapsed-time-without-code.aspx (((substring-before(endTime, ":") * 60) + substring-before(substring-after(endTime, ":"), ":")) - ((substring-before(startTime, ":") * 60) + ...

I have two drop down fields data type time named TimeIn and TimeOut and another text box field named hours that I want to show the results of the time a user worked.Example:TimeIn = 9:00 AM TimeOut = 5:00 PM then hours = 8I can't figure how in infopath to calculate the hours field, any ideas on how to solve this issue?