function onEdit(e) {
  if (e.range.columnStart != 2) return; // if it's not column B do nothing

  var date = e.range.getValue();

  if (date.getFullYear() < 1990) {
    const [h,m,s] = ['Hours','Minutes','Seconds'].map(s => date[`get${s}`]());
    e.range.setValue(new Date(new Date().setHours(h,m,s,0)));
  }

  e.range.setNumberFormat('hh:mm am/pm');
}