While you can't directly sync the Psylio calendar with an external one, you can export your Psylio calendar to view your scheduled client appointments in an external calendar.
You can also access this option by going to the Settings tab and selecting Calendar export.

Important:
- The export process may take a few minutes to a few hours, depending on the external calendar used.
- You cannot import an external calendar into Psylio. The export feature only works one way.