VBA for time calculation

[Sods Law]

Good morning,

I've been using MrExcel for ove a year, it's always managed to awnser questions in some form or another.

This one has me stumped I need to subtract the time between finished time and started to get total time taken. I know how to do the =H2-H1, but the table is going to be rather long so doing this manually will take a lot of time.
I'm not going to be the end user so being able to press a button and have it automatically do this would be a great help.

A little info about the excel sheet, I have created it using power query and a pivot table to get a usable end result. I have attached an image that can sa a thousand words

Thanks in advanced for any help. Apologies if I have not provided enough information.

 

[Sods Law]

I forgot to say, I think it would be easier if it was a button that when clicked it asked for first cell reference and then second cell reference and then subtracted the selected cell values.

Sorry I have no code at he moment to post, as I am drawing a blank on how to start this one.

 

[Sods Law]

Good afternoon,

I have been editing the above for a littel while and believe I may have an easier solution, I just don't now how to filter the index column to keep the min and max values.

I would like to keep the 1's and then the highest value for each reference in column A.

any help would be greatly appreciated.