UWP apps use the new CoreCLR for compilation. The CoreCLR does not support
DateTime.ToShortDateString at this time, and it doesn't seem to be getting much traction anyways. Those helper methods are bad at localization.
You can simply replace the call with
Looks like it got some traction! This method (along with several others) was added back into the CoreCLR. This move was done to improve parity with the APIs in Xamarin and the .Net Framework. You can read more about this decision here.