In many cases, it’s because your ‘Sync’ setting of the standard iOS Calendar is still set to the default value, “Events 1 Month Back”. Calendar Op.2 shares your calendar data with standard iOS Calendar app, so you need to change the setting of the standard iOS Calendar in order to make your old events visible on your device by following the steps below.

  1. Launch ‘Settings’ app from the home screen.
  2. Go to ‘Calendar’ > ‘Sync’ in the Settings app.
  3. Select “All Events”.