// [SOLVED]  var date  = new Date(year + 1, month, day);

$('#membership_start').on('dp.change', function(e){
   console.log(e.date);

   var year  = new Date(e.date).getFullYear();
   var month = new Date(e.date).getMonth();
   var day   = new Date(e.date).getDate();
   var date  = new Date(year + 1, month, day);
   $('#membership_end').data("DateTimePicker").date(date);
 });