PERIFACT has 2 calendars/calendars built in by default, one is called "Appointments" and the other "Run data".
You can use both calendars to include own (private) appointments. However, PERIFACT will update the calendar with "Run data" use to automatic notes create, it looks at the FALSE DATE, look at the [Characteristics] tab when creating an invoice:
In the menu SETTINGS - EXTRA, tab [Calendar] you can delete appointments in 1 go. But also add them again automatically, in the latter case all invoices and quotations are run through and added back to your calendar.