Here’s a link with one way to do this:
https://community.powerbi.com/t5/Desktop/Get-start-and-end-date-from-Timeline-Slicer/td-p/71317
Basically, just MIN and MAX of ALLSELECTED on your date field.
SELECTEDVALUE returns a blank since it doesn’t know which of the two dates to select (to me, it would make more sense if it returned an error message instead, noting the ambiguity).
Hope this is helpful.
- Brian