Problem in change date format calendar

Problem in change date format calendar

I have an issue when I change date formet  it says Invalid date formet

my code is 

var startdate = new Date($("#datepicker1").val());
  alert(startdate);
    var dateToday = new Date();
alert(dateToday);
    var dates = $(".date-picker").datepicker({
defaultDate: "+1w",
changeMonth: true,
numberOfMonths: 1,
minDate: dateToday,
    dateFormat: 'dd-mm-yy'
    });