Suppose a problem has three time points, T0, T1 and T2.
Then the duration of T0 to T2 is equation to the sum of the duration of
T0 to T1 and the duration of T1 to T2. That is, if the duration variables
are T02, T01 and T12, then T02=T01+T12.
If you are seeking one of these durations, then you can select "Sum
of times" if you want to use this equation to find it.