Example: The times T1 and T2 are given by T1 = 6 hours 34 minutes 56 seconds and T2 = 2 hours 51 minutes 45 seconds.
Add times
Let S = T1 + T2 = (6 hours 34 minutes 56 seconds) + (2 hours 51 minutes 45 seconds)
add hours together, minutes together and seconds together
S = (6 + 2) hours (34 + 51) minutes (56 + 45) seconds = 8 hours 85 minutes 101 seconds
We now need to convert 85 minutes to hours : 86 minutes = 1 hour 25 minutes and 101 seconds to minutes: 101 seconds = 1 minute 41 seconds; and substitute
S = 8 hours 85 minutes 101 seconds = 8 hours + (1 hour 25 minutes) + (1 minute 41 seconds) = 9 hours 26 minutes 41 seconds

Subtract times
With the subtraction, it might be easier to convert each time into seconds, subtract and convert again into hours, minutes and seconds.
T1 = 6 hours 34 minutes 56 seconds = 6 × 3600 seconds + 34 × 60 seconds + 56 seconds = 23696 seconds
T2 = 2 hours 51 minutes 45 seconds = 2 × 3600 seconds + 51 × 60 seconds + 45 seconds = 10305 seconds
Let D = T1 - T2 = 23696 seconds - 10305 seconds = 13391 seconds
Convert 13391 seconds into hours
D = 13391 seconds / 3600 = 3.71972222222 hours = 3 hours + 0.71972222222 × 60 minutes = 3 hours 43.1833333333 minutes = 3 hours 43 minutes 0.1833333333 × 60 seconds = 3 hours 43 minutes 11 seconds

How to Use the Time Calculator

1 - Enter the times T1 and T2 in hours, minutes and seconds form, check your input and press "Add / Subtract". The outputs are the sum T1 + T2 and the difference T1 - T2 in two forms: hours, minutes, seconds and also in decimal hours.
Example of times T1 and T2
T1 = 6 hours 34 minutes 56 seconds
T2 = 2 hours 51 minutes 45 seconds