Discount count based on 15 minute interval

Hi all,

Below is a sample of the data that was given to me.  The goal (which seems pretty simple) is to count the number of times a customer opened an email sent to them using the Opened On date/time stamp.  The crux is there has to be a 15-min interval between the opened times.  

So in the example below, Customer A shows they opened the email 5x on 11/14 but it should only be counted 3x.  The first and second time it was opened (at 9:37am and 9:38am) only counts once and the lat two times (at 11:56 and 11:56 AM) also only counts once.    Customers B and C would have longer intervals between the times they opened the email so each one would be counted.

Do I need to create a date/time table first to get the intervals? 

Thank you for any help in advance.