Calculating total time based on changes in another column

I have two columns with date and times. A third column that indicates a step number that a machine was in during the date and time in the same row. The step numbers change as time progresses. I want to calculate the total time that the machine was in each step with out manually entering a function at the last date/entry for each step.

The data set has over 70K rows. I would like to create one function and copy through out the entire column.

Thanks