InfoPath Dev

I've removed the = > 0 and found that I am now returning a numerical value but can easily tell that what should have come back at 30 seconds returns 60.5 so again I'm getting closer but still unsure as to exactly what my mistake is.  I'm guesing it has something to do with converting hours and minutes into seconds.

I've gotten far evnough that I feel like I'm learning somthing but I'm still unsure about the end result. I now have the following formula retrning something. (number(substring(txtEnd_Time, 12, 2)) * 3600 + number(substring(txtEnd_Time, 15, 2)) * 60 + number(substring(txtEnd_Time, 18, 2)) - (number(substring(txtDateTime, 12, 2)) ...

I have created a form that uses the now() function to populate the begin time of a task and also added a rule to a submit button that timestamps the completion of the task using the now() function. I need to be able to determine the difference between the start time and the end time in order to start gather data on the category and types of tasks ...