With Redtail's Calendar, you have the option to set your default Start and End Times, so that you do not display times outside of when you schedule activities.
To do so, you'll need to access your Site Preferences from the menu accessible when clicking your name in the upper right corner of your database:
On the resulting page you can set these within your Activity Management section:
Click within each of these fields to select your desired Start and End Times for your Calendar.
After doing so, click Save Preferences in the upper right hand corner of the Activity Management section and you will have successfully changed your Default Calendar timeframe.