Hello Bill Spaulding,
The RadSlider support SelectionRange as you described in your requirements, it can be enabled by setting the IsSelectionRangeEnabled property to "True", for more information you can check this article
. The ticks of the RadSlider also can be customized using the TickTemplate property and through converter to make the tick value to be displayed as a Date.
I have attached a sample project demonstrating the above approach for your convenience.
Also I am not sure what you mean by "a way to add multiple thumbs beyond a range". Do you expect to have more than two thumbs? If you can provide us with a picture showing how you expect the control to look like, will help us better understand your scenario.