validating date not working

i have a validation rule for date as follows

 'date' => 'required|date|date_format:"d/m/Y"',

and i have a jquery date picker on the page and it shows the date

with a format

      $('.datepicker').datepicker({
         dateFormat: 'dd/mm/yy'
       });

and show the date like this

12/09/2016

when i enter date like the above it saves well and when i just choose another date 13/09/2016 it return error says The date is not a valid date !!!

although the same code without any change !!! but just a different date