Calculating time between two timestamps

[rfinnegan]

Hi All:

I'm trying to calculate the average transaction time and need some help.
The data is arranged with the transaction number in column A and the time each item in the transaction was scanned in column B (think supermarket scanner).

For example if I had a transaction with 8 items, the transaction number would repeat 8 times in column A, and there would be 8 separate time stamps in Column B.

I need a way to find the difference between the first time stamp and the last for each transaction number. There's about 50K lines of data and each transaction has between 1 and 40 items.

Thanks in advance

 

[DanteAmor]

It would be very useful if you explain all the possible cases with examples. In that way we will create a formula that covers all cases.