How it works
Each duration is converted to total seconds, combined, and then converted back to hours, minutes, and seconds. Because there are 60 seconds in a minute and 60 minutes in an hour, any overflow carries up cleanly.
total seconds = h×3600 + m×60 + s
decimal hours = total seconds ÷ 3600
decimal hours = total seconds ÷ 3600
Worked example
2:45:30 plus 1:30:45 is 9,930 + 5,445 = 15,375 seconds, which is 4 hours, 16 minutes, and 15 seconds — or 4.27 decimal hours.