Have two global variables. The first one accumulates the total time taken so far. The second one calculating the difference between the current action. Then do something like this.... (This is psuedo-code, so you will have to tweak it to make it actually work).
Global NumberVar TotalTime
Global DateTime ActionTimeStart
if {Table.Field} = 'action begin' then
//start the timer
ActionTimeStart = {Table.DateField};
else
/calculate total time elapsed and save it
TotalTime = TotalTime + {Table.DateField} - ActionTimeStart;
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