When creating a date slicer, the input boxes allow the user to pick any date regardless if it is in the range or not.
Create a calendar tabel with Calendar = CALENDAR (DATE(2015, 1, 1), DATE(2018, 12, 31))
Create a slicer visualization with the calendar as its field
The slider portion of the visualization (if turned on) only shows the appropraite range. However, the sclier input boxes allow any date to be selected anyway. If a date outside the range is selected the slider enlarges as well. I believe this is a bug/oversight in the design of the visual.
1) Prevent the popup calendars from showing dates outside the range
2) Default to the minimum date if a date before then is selected. Default to the maximum date if a date above it is selected.