Calendar integration allows you to place tasks on your device's calendar.  This gives you the option of viewing both your tasks and calendar events in one place - using the built-in calendar software on your device.

Calendar integration is an optional feature, which can be turned off if desired.

You can optionally set up the app to automatically add any new tasks to your calendar.  You can also choose which calendar the tasks are added to.

Ultimate To-Do List maintains a link between your tasks and their associated calendar events.  This means that changes made to the tasks will automatically appear on the calendar as well.  When viewing the event in your calendar, you will see a link you can tap on to go to the associated task.  Keep in mind that you should not edit linked calendar events using your calendar software, as the changes will not be reflected in Ultimate To-Do List.