Introduction

Google lately added the Google Apps Script functionality in Google Documents and presented some basics on the Google Developers site.

One key feature is the sidebar. This is where the "Quick Parts" could reside. It needs however a bit of coding to do that. See the following example, made by Martin Hawksey: Sidebar

Update 27/07/2013, here's an example I wrote myself:

Code

// global
var app = DocumentApp.getUi();

function onOpen() {
  app.createMenu('Quick Parts')
    .addItem('Document Property', 'docProperty').addToUi();
}

function docProperty() {
  // set variables
  var doc = DocumentApp.getActiveDocument(); 
  var fileName = doc.getName(), Id = doc.getId();
  var file = DocsList.getFileById(Id), lastUpdated = file.getLastUpdated();
  var fileOwner = file.getOwner().getEmail();

  // arrays with label and result names
  var aNames = ['File name', 'File Id', 'File Owner', 'Last updated by'];
  var aResults = [fileName, Id, fileOwner, lastUpdated];

  // create Ui  
  var Ui = UiApp.createApplication().setTitle('Quick Parts').setWidth(450);
  var vPanel = Ui.createVerticalPanel().setId('vPanel').setSize(450, 100);  
  var fTable = Ui.createFlexTable()
    .setStyleAttribute('borderCollapse','collapse');

  // create labels
  for(var i=0, iLen=aNames.length; i<iLen; i++) {
    fTable.setWidget(i, 0, Ui.createLabel(aNames[i]));
    fTable.setWidget(i, 1, Ui.createLabel(aResults[i]));
  }

  // add to Ui
  vPanel.add(Ui.createLabel().setText("Document Property")
    .setStyleAttribute('font-size','175%'));
  vPanel.add(fTable);
  app.showSidebar(Ui.add(vPanel));
}

Screenshot

Short answer

At this time, variables is not a built-in feature of Google Docs and Google Apps Script, the platform to extend Google Docs, does not include a class or method to handle them.

Alternatives

Alternative 1

One alternative is use a text pattern but you should be sure that it will match only the date that you want to update.

Alternative 2

Another alternative is to use the class NamedRange but bear in mind that

1. moving the range will make that it lose its name¹.
2. replacing text in a named range with multiple elements only works the first time².

Code:

The following code, intended to be used in a script bound to a Google Document, has two main functions:

1. Insert today's date
2. Update today's date

For debugging purposes are being used

1. date and time, instead of date only.
2. custom menus to trigger the main functions.

"Known-issues": The update function replaces the whole paragraph.

To test the code, copy it, then go to your Google Docs, create a new document, click on Tools > Script Editor, select Blank Project, paste the code, save the project, assign a name, run on time to authorize the app, close your doc and open again. A new menu called "Utilities" will be displayed. From there you can call the main functions.

function onOpen() {
  // Add a menu with some items, some separators, and a sub-menu.
  DocumentApp.getUi().createMenu('Utilities')
      .addItem('Insert Today\'s Date', 'insertTodayAtCursor')
      .addItem('Update Today\'s Date', 'setTodayNamedRange')
      .addToUi();
}

function todayDate(){
  return Utilities.formatDate(new Date(), "GMT-5", "yyyy-MM-dd'T'HH:mm:ss'Z'"); // "yyyy-MM-dd"
}

/**
 * Inserts the today's date at the current cursor location and create a NamedRange.
 */
function insertTodayAtCursor() {
  var str = 'testToday';
  var doc = DocumentApp.getActiveDocument();
  var cursor = doc.getCursor();

  if (cursor) {
    // Attempt to insert today's date at the cursor position. If insertion returns null,
    // then the cursor's containing element doesn't allow text insertions.
    var date = todayDate();
    var element = cursor.insertText(date);
    if (element) {
      var rangeBuilder = doc.newRange();
      rangeBuilder.addElement(element);
      return doc.addNamedRange(str, rangeBuilder.build()); 
    } else {
      DocumentApp.getUi().alert('Cannot insert text at this cursor location.');
    }
  } else {
    DocumentApp.getUi().alert('Cannot find a cursor in the document.');
  }
}

function setTodayNamedRange(){
  var str = 'testToday';
  var doc = DocumentApp.getActiveDocument();
  // Retrieve the named range
  var namedRanges = doc.getNamedRanges();
  var newRange = doc.newRange();
  var date = todayDate();
  for(var i=0; i<namedRanges.length; i++){

    if(namedRanges[i].getName() == str){

      var rangeElement = namedRanges[i].getRange().getRangeElements();

      for (var j=0; j<rangeElement.length; j++){

        var element = rangeElement[j].getElement().asText().editAsText().setText(date);
        newRange.addElement(element);
      }
    }
  }
  doc.addNamedRange(str, newRange.build());
}

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Below there are some items of different kind (questions, specifications, etc.) that could serve to get inspiration or point to the "right direction" to find a "solution"

• Class NamedRange - Document - Google Apps Script
• How to use method replaceText(searchPattern, replacement) in documents Service/ text Class - Stack Overflow
• Update/Replace inline image on Google Document - Stack Overflow
• Google App Scripts for Documents: insert text and add it to a named range for later retrieval - Stack Overflow
• Adding custom attributes to google doc text - Stack Overflow

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Footnotes

• ¹: Issue 3831: Preserve NamedRanges in Docs during moving/cut-pasting
• ²: Issue 3766: Document App Named Ranges not working as expected