Correct me if I understand this right again,
For a given Ticket 123, you have multiple states i.e. in this case you are calculating the time based on the time taken from one status to another , am I right?
Stage 1: Let say 'Open' to 'Acknowledge' you have taken 2 mins - which field captures this time? Lets say X
Stage 2: Next 'Acknowledge' to 'In Progress' you have taken another 2 mins - which field captures this time? Lets say Y
Step3: If you want an combined time you may want to Add both X + Y.
But I guess it all depends on what field is used to capture the data. You might as well need couple of fields published so that one will hold the current time and the other will hold the cumulative time of all the stages.
You can always add the current time to cumulative and show the data accordingly. Hope My logic is in sync with yours.