Hi

Yes

Set one data field to have its default vaule = Now() untick the this value
is recalculated and that is your start time and date.  On the submit button
have rule the sets the other data field to Now() and untick the same
recalculate box.

Thus you have a start and end time recorded.  Now to calculate time
differences is a tricky funtion and i have never bothered learning it.  
However, there are one or two complete descriptions of how to do it on this
forum that you should take a look at.

Cheers CG

hope this helps Adam

Adam's suggestion should work. I have just one addition: After you have the
two fields as indicated by Adam, you can use a formula as demonstrated in
this article
http://enterprise-solutions.swits.net/infopath/time-calculations-using-rules.htm
to calculate the difference in time. Since your users aren't expected to take
more than one day to fill out the form, it should be enough to cover your
scenario.

A second option is to use code in the OnLoad and OnSubmitRequest events of
the form to do the initialization of the timer and saving of the time elapsed.
---
S.Y.M. Wong-A-Ton