I have the following spreadsheet

Customer ID Visit ID Day of week Visit start time Visit
end time
-------------- -------- -------------- -----------------
----------------
0001 01 Mon 9:00
10:00
0001 02 Tue 9:00
9:30
0002 03 Tue 9:15
11:30
0003 04 Mon 9:30
11:00
etc...

What I want to do is calculate the number of visits that occur for each 15
minute block of time throughout the week. For example, and using the four
lines above: -
....
Mon 9:00 to 9:15 = 1 visit
Mon 9:15 to 9:30 = 0 visit
Mon 9:30 to 9:45 = 1 visit
Mon 9:45 to 10:00 = 1 visit
....
Tue 9:00 to 9:15 = 1 visit
Tue 9:15 to 9:30 = 2 visit
Tue 9:30 to 9:45 = 1 visits
Tue 9:45 to 10:00 = 1 visit
....

Your advice would be much appreciated.