With Cal, you can choose to have custom time-slot intervals. By default, a time-slot interval is of the same length as the event type. However, within each individual event-type, you have the option to choose a different time-slot interval. Here's how to do that:
Select your event type where you wish to customise the time-slot interval
To change the time-slot interval, click on the Limits tab
Then, click on the time-slot interval select box to view the interval options in minutes and select the time interval of your choice
Click on Update to save your changes. Your event type should now consider the newly added time-slot interval