addTimestamp.gs

function onEdit() {

    var doc, footer, style = {};

    // Define a custom paragraph style.
    style[DocumentApp.Attribute.HORIZONTAL_ALIGNMENT] = DocumentApp.HorizontalAlignment.RIGHT;
    style[DocumentApp.Attribute.FONT_FAMILY] = 'Proxima Nova';
    style[DocumentApp.Attribute.FONT_SIZE] = 6;
    style[DocumentApp.Attribute.BOLD] = false;
    style[DocumentApp.Attribute.ITALIC] = true;

    doc = DocumentApp.getActiveDocument();
    footer = doc.getFooter();

    var d = new Date();
    var utc = new Date(d.getTime() + d.getTimezoneOffset() * 60000);

    if (!footer) {
        doc.addFooter();
    }

    footer.removeChild(footer.getChild(0));
    footer = footer.insertParagraph(0, 'Revised ' + utc);

    footer.setAttributes(style).setForegroundColor('#999999');

}