In one-way sync mode, you can connect any web calendars in the iCal format: events from these calendars cannot be checked off or edited, but they can be viewed as a list on the Today tab.
Two-way synchronization is available in the Pro and Elite versions of SingularityApp and only for Google Calendar. When two-way sync is set up, all events from the external calendar are fully synchronized with tasks in the app. You can edit, complete, or delete them either in the app or in Google — changes instantly appear in both places. Learn more about two-way synchronization in the blog and SingularityApp Wiki.