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The default spark checkbox extends from ToggleButtonBase which has 2 selection states, selected or not selected. But since we are dealing with 3 state checkbox, we need more. So we are going to make a checkbox extended from ButtonBase which is the base class of most button controls.
Extend the List control to customize the behavior of multiple selection.
We create a custom parseFunction that will return a Date object. We will still follow the arrangement from the assigned format but let users type common formats of date. Also we are going to need a function that returns the index of a string representation of a month.