You are able to get a list of pets who have had a grooming appointment since X date, and a list of pets who have not had an appointment since X day.

Click on Reports, Grooming, Last in report.

Pets In Since will give you the list of pets who have had an appointment.

Pets Not In Since will give you the list of pets who have not had an appointment.

If you check the box for Checked in pets, the pet has to have been checked in for this report.

If you leave the box for Checked in pets unchecked, the pet just needs to have an appointment for the date range. 

This report is printed by the client name.  It will show the client name, email address, phone number and last date their pet(s) were there.