If it is not working you may have installed the JavaScript file in the wrong directory or folder since the User's application JavaScript folder changes with each new version.
For Windows:
C:\Users\JoeUser\AppData\Roaming\Adobe\Acrobat\Privileged\DC\JavaScripts
of
C:\Users\JoeUser\AppData\Roaming\Adobe\Acrobat\Privileged\DC\2015\JavaScripts
You need to change "JowUser" to your loginID.
You may need to create the "Privileged", "DC", and "2015" folders.
This script is also only for Acrobat DC and not Adobe Reader DC.
As noted FormRouter offers the tool bar and PDFscripting.com also offers the same pop-up calendar insertion tool. FormRouter's version has August with 30 days and not 31 days, this can be fixed by reviewing the code and for the month/day array changing the entry for the number of days in August from 30 to 31. PDFScripting.com does not have this coding error.