From the searches I've done, the Date Picker Extension is proving to not be reliable.
At first I was not able to select a date. I used the link below to modify the load script for the date field I'm using. That allowed me to choose a date, but I am still having an issue.
https://community.qlik.com/t5/New-to-Qlik-Sense/Date-picker-not-working-with-my-date-field/m-p/16633...
The picker shifts to the next available date.
In our case all of our dates are month end dates. Upon providing the dates, the date that returns is the next month, not the month selected.