After updating Joomla to version 3.6.2 and K2Store to 3.8.3 the 'enable K2Store Cart' was no longer working in a K2 Item and the date selector would not pop up either. Turns out that there was a jquery conflict.

What fixed it for me was to go to K2Store / Options / and set the Load Jquery UI to Only Frontend.

This may also help with problems selecting an author and enabling or disabling an article with the "Red and Green" Buttons.

Thank you Robo for letting the community know.
If possible post this to the K2Store forums so their devs know about this.