You can schedule clients at any time increment in TheraNest! You may need to adjust your calendar preferences to allow for this.
In the upper right click on your name and then click calendar preferences. Scroll down and next to "Time scale in minutes", change the increments. For example, setting your time scale units for 15 minutes allows you to schedule an appointment using 15 minute increments.
https://theranest.zendesk.com/hc/en-us/articles/360052498031-Set-Up-Calendar-Appearances-Preferences
If this isn't what you were referring to, please resubmit your idea with as much detail as possible. Thank you!