Change these two lines.

    minSplit = minimum.split("-");
    var newMin = (minSplit[0]+"-"+minSplit[1]+"-"+minSplit[2]);

Additionally, you should disallow direct typing in the box with something like this:

   $(document).on('keydown', '.DateFrom input, .DateTo input', pickerOnly);
  
   function pickerOnly (e) {
   e.preventDefault();
   return false;
}

Complete code should look something like this:

$(document).ready(function () { 

$(document).on('keydown', '.DateFrom input, .DateTo input', pickerOnly);
  
   function pickerOnly (e) {
   e.preventDefault();
   return false;
}
  
    function updateMinimumEndDate () 
  {
    var minimum = $('.DateFrom input').val();
    var minSplit = [];
    minSplit = minimum.split("-");
    var newMin = (minSplit[0]+"-"+minSplit[1]+"-"+minSplit[2]);
    $('.DateTo input').attr('min',newMin);

  }
  $('.DateFrom input').change(updateMinimumEndDate);

});