There was a bug in Joomla 3.7.0 that prevented the date picker working in the back end of Rentalot, Rentalot Plus and MediaShop, and in the front end of Rentalot Plus and FlexiContact Plus if you selected the Joomla date picker.

The problem was fixed in Joomla 3.7.1 so the solution is to upgrade to the latest version of Joomla.