With Redtail's Calendar, you have the option to set your viewable time increments in 15 minute, 30 minute or 60 minute slots.
To set these, go to your Preferences:
The second section you'll see on the Preferences page is Activity Management:
Click inside the Default Calendar Time Slots box to select from your available options:
After selecting your desired Calendar Time Slots, click Save Preferences in the upper right hand corner of the Activity Management section and you will have successfully changed these.