Date composition from day, month and year input
As Ekkhard.Homer said, none of you delete lines do anything, because startDate and endDate are strings at that point: They don't have any day, month or year properties.

Anyway, since you need to do the same thing twice, I'd suggest you make a function for it. I've spruced this one up a bit by mapping properties, but that's not really necessary.

$scope.addWeek = function(week) {

  function formatDate(obj) {
    return ['day', 'month', 'year'].map(function (prop) {
      return String(obj[prop]);
    }).join('-');
  }

  week.startDate = formatDate(week.startDate);
  week.endDate = formatDate(week.endDate);
}