Grouping log in times into 15 minute intervals

I have been given the task of finding out who logs in at what time of the day.

I have attached the raw data that I have to work with.

In essence I need to see how many login's there were for 06.00 - 10.00 at 15 minute intervals e.g:

06:00 - 5 Logins
06:15 - 1 Logins
06:30 - 3 Logins
06:45 - 11 Logins
etc

Has anyone got any suggestions?