Keeping track of appointments in the calendar?

Now that there is a daily receipts book, is it still necessary to link the receipts to appointments in my calendar?

Up to now, I’ve been filling in the daily receipts book when there’s no invoice, keeping track in an Excel file of which session corresponds to which payment, and noting in a physical diary the date I worked with which customer.
I work with programmes. A customer pays for 6 hours of support, and I naturally keep track of when those hours were used so that I know how much a customer still has left and when the programme has ended.

A former colleague (an accountant) once advised me to also keep a physical diary so that I can link each payment to an appointment.
Is it still necessary to write appointments in a physical diary?

I do use Calendly, but if I ever stop the subscription, the evidence will be lost.