Setting a Timezone for Google Calendar Integration:
Google Calendar API needs a valid timezone to add your appointments in your Google Calendar. This is a common issue with new WordPress installations, sometimes the users don’t set a timezone which causes an error message when the client submits a new booking on the front-end booking form.
This can be resolved by clicking on WP Settings – General, you will see the following screen. Just set your timezone and save the change.
IMPORTANT: PLEASE READ: The Time format should be for example : “America/Chicago”
Do not use UTC timezones, this is not allowed by Google Calendar.