How do i compute for the time difference of a certain change in another column with reference to a specific name

How do i compute for the time difference of a certain change in another column with reference to a specific name. See example below. I have my date and time in column 1 and in column 2 i have my alarm type that changes in a specific given time in column 3 i have my Pointname with different alarm type listed on column two, i just i want to know how do i calculate the change of time in every change of alarmtype of a certain pointname
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Author: pags