You can connect an external calendar via CalDav.
To do that, go to the Calendar section > Synchronize.
Click the Connect other calendars button.
Click the External Calendar button:
Specify here:
- the calendar name to display in your calendars list.
- the calendar server address.
- login and password.
Click Connect. If everything is correct, the calendar list will show the external calendar connected.
Please note that though connected external calendar events will appear in your Bitrix24, the event reminders will not.