Insert dd-mm-yyyy date in Google Doc

insert-date.gs

function onInstall(e) {
  onOpen(e);
}

function onOpen() {
  DocumentApp.getUi().createMenu('Utilities')
      .addItem('Insert today\'s date', 'insertDate')
      .addToUi();
}

function insertDate() {
  var cursor = DocumentApp.getActiveDocument().getCursor();
  var selection = DocumentApp.getActiveDocument().getSelection();
  var date = Utilities.formatDate(new Date(), "GMT", "dd-MM-yyyy");
  if (cursor) {    
    var element = cursor.insertText(date);
  } else if (selection) {
    replaceSelection(date);
  } else {
    DocumentApp.getUi().alert('Error: could not insert date');
  }
}

// https://stackoverflow.com/a/21613263
function replaceSelection(newText) {
  var selection = DocumentApp.getActiveDocument().getSelection();
  if (selection) {
    var elements = selection.getRangeElements();
    var replace = true;
    for (var i = 0; i < elements.length; i++) {
      if (elements[i].isPartial()) {
        var element = elements[i].getElement().asText();
        var startIndex = elements[i].getStartOffset();
        var endIndex = elements[i].getEndOffsetInclusive();
        var text = element.getText().substring(startIndex, endIndex + 1);
        element.deleteText(startIndex, endIndex);
        if( replace ) {
          element.insertText(startIndex, newText);
          replace = false;
        }
      } else {
        var element = elements[i].getElement();
        if( replace && element.editAsText ) {
          element.clear().asText().setText(newText);
          replace = false;
        } else {
          if( replace && i === elements.length -1 ) {
            var parent = element.getParent();
            parent[parent.insertText ? 'insertText' : 'insertParagraph'](parent.getChildIndex(element), newText);
            replace = false; //not really necessary since it's the last one
          }
          element.removeFromParent();
        }
      }
    }
  } else
    throw "Hey, select something so I can replace!";
}