Sorry error in first post.

MGuy's code above makes sense to me. I believe (but could be wrong) that his code makes new time cards and simply adds them together for a sum of total duration. (via a parent relationship)

Either way you will need to create a list of duration values to add together into a total duration field. I was thinking that the sys_audit table would hold this list of duration values but creating new time cards is a simpler and more elegant solution.

My apologies, I can understand the process but I'm not a very good coder so I can't help you there.