The total time pops in as soon as I enter a start time. It calculates time from zero hour to the start time (start time of 4:15 PM leads to total time of -16:15
That sounds like expected behavior. If this is a calculation field, entering a start time with no end time will result in a negative elapsed time as an empty field will evaluate as though you had a zero in the field.
I cannot get the end time to insert at all.
How are you setting up your button and the end time field to do this?
You mentioned IWP. Does this work correctly in FileMaker, but not in when you use a Browser to access the database or does it fail in both cases?