Add date to IPTC Headline

You could try this....

#target bridge   
if( BridgeTalk.appName == "bridge" ) {  
updateHeadline = new MenuElement("command", "Add Date to Headline",  "at the end of Tools","adtoh");
}
updateHeadline.onSelect = function () {
var sels = app.document.selections;  
if (ExternalObject.AdobeXMPScript == undefined) ExternalObject.AdobeXMPScript = new ExternalObject("lib:AdobeXMPScript");
for(var a in sels){
var thumb = new Thumbnail(sels); 
var myXmpFile = new XMPFile(thumb.spec.fsName, XMPConst.UNKNOWN, XMPConst.OPEN_FOR_UPDATE);  
var xmp = myXmpFile.getXMP();   
var DateCreated = xmp.getProperty(XMPConst.NS_PHOTOSHOP, "DateCreated");
var newDate = new Date(new XMPDateTime(DateCreated.toString()).getDate().getTime());
var Year =newDate.getFullYear(); 
var Month = newDate.getMonth() + 1;
if(Month < 10) Month = "0" + Month;
var Day  = newDate.getDate();
if(Day < 10) Day = "0" + Day;
var DateNow = Day + "/" + Month + "/" + Year;
var Head = xmp.getProperty(XMPConst.NS_PHOTOSHOP, "Headline");
Head += " " + DateNow;
xmp.deleteProperty(XMPConst.NS_PHOTOSHOP, "Headline"); 
xmp.setProperty(XMPConst.NS_PHOTOSHOP, "Headline", Head); 
if (myXmpFile.canPutXMP(xmp)) { 
          try{
            myXmpFile.putXMP(xmp);
            myXmpFile.closeFile(XMPConst.CLOSE_UPDATE_SAFELY); 
            }catch(e){
                myXmpFile.closeFile();
                }
            }
    }
};