Etar works as a view onto the calendars already available through Android. Treat account synchronization as a separate layer, then choose the day, week, month or agenda view that best supports your daily review.
First configure calendar synchronization at the Android account level, then select the calendars you want Etar to display. Create a test event and check that it appears on another device before relying on the setup. Use agenda view for a compact review of upcoming commitments.