Pocket Lists supports realtime import from the Apple's basic 
Calendar app.
The import is updated every time you launch the app and every time the iOS throws an event that a new event/task has been added on the Calendar app side.
Pocket Lists only reads data from the Calendar, and does not send anything back (except for a checkmark symbol for completed to-dos; but that can be easily turned on/off in the app settings). It means that you may freely edit and delete Pocket Lists to-dos that were originally imported from the Calendar, and your Calendar app events won't be affected.
Head on to Settings → Import → Calendar to configure the import and limit the list of calendars to be imported.
Feel free to disable and re-enable the import any time. It will only affect the import auto-update feature. All the data that were previously imported will stay in the Pocket Lists app unless you manually delete it.
Google Calendar & Outlook
Pocket Lists does not import anything directly from the Google Calendar and Outlook. However, if any of these calendars are linked with the default iOS Calendar app, then their events will eventually be imported into Pocket Lists.