Calculating time of a specific activity

Hi there, I was wondering if there is a simple VBA/formula to calculate the total time of a specific activity (differentiated by ID)?
To better explain please refer to the attached excel file.

for example activity id 1, there could be many sub tasks until it is reported as 1 (column C). The total time will be calculated based on the same activity but only for the tasks before it gets reported (column c).

I have to mention that the file I am working on is huge so I'm not sure if the use of formulas would be a good idea.

Appreciate your input.

Thanks in advance.