Hi Jim, that behaviour can be done with extensions, here are some that can do the task:
As I know, this setting is not at Qlik Sense. But you can try a workaround like this (the attached file) - in the filter box enter the expression:
If(GetSelectedCount([%Date])=0 Or GetSelectedCount([%Date])=1, Date([%Date]), 'Select only one value')
Not ideal, but somehow it works. Maybe in your case suitable.
Temp_Other.qvf 176.0 K