Ok, this might be hard to understand, but here goes;
Cell 1 - Your start time 4:11 (btw with the ":" is how to do time in numerals for this)
Cell 2 - Now have another cell with the time to be added (3:43)
Now cells 1 & you addition cells should be in time format shown as 1.30 PM (the second time format style), with cell 2 to just in time format 13.30 (the first one shown in format styles).
So with the first addition if you have the format =cell1+cell2 (make sure it's (and all subsequence cells) in 1.30 pm time format.
picture link:
note that the cell with 3:43 is in 13.30 format.
If you want time to start from 4.11 pm make the time in 24hrs ie: 16:11 (again make sure it's in 1.30 pm format)
Hope this helps