There is a shipping example called "NET Event Callback for Calendar Control". This shows you what you essentially have to do: you need to register a callback for the calendar. In this example when the user clicks on a date the callback VI updates the timestamp indicator to the selected date. You would do something similar. The callback VI provides a reference for the .NET calendar, so you can access the properties. The SelectionStart property requires a DateTime object, as explained here. You can create DateTime objects using a DateTime constructor (via the LabVIEW .NET constructor node).
There will be some examples for .NET controls on the NI site, but for specific issues with .NET controls the best place to look is outside of NI, since .NET is not distributed by NI.
