var currentdate = 0;
var core = 0;
var ran = 60/6;

function ranimage() {
  currentdate = new Date();
  core = currentdate.getSeconds();
  core = Math.floor(core/ran)+1;
  return("<img src=\"images/banner."+core+".jpg\" alt=\"Isis Construction - Building confidence - Newquay, Cornwall, "+core+"\">");
}

document.write(ranimage());