function MostraPeriodoDoDia()
{

  var dhoje= new Date();
  var hh=dhoje.getHours();
  var dname="";

  if ((hh>=0)&&(hh<12)) dname = "Bom dia";
  if ((hh>=12)&&(hh<18)) dname = "Boa tarde";
  if (hh>=18) dname = "Boa noite";

  document.write(dname);

}