[Q] date-time substraction for hours

I need to know if there is a way to do the following in SQL. I have a historical database for visitors. Every time an entry is made to the system, the following fields are inserted into my Hist_visitors table, lname, fname, vendor, date_time_in, status Lennon, John, Beatles, 1996-03-01 10:41 PM, IN and on exiting the site Lennon, John, Beatles, 1996-03-02 2:30 AM, OUT My question is for finding out the number of man hours spend by visitors inside our site - for every day, allowing for repeat visitors. I have a couple of ideas but nothing concrete. If someone has done anything like this before I would love to hear their suggestions. -- Saqib Mausoof -------------------------------------------------------------------- I have a hard enough time speaking for myself, so speaking on behalf of my employer is out of the question.