onEdit

function onEdit(event)
{
  var ss        = event.source.getActiveSheet();
  var r         = event.source.getActiveRange();
  var today     = new Date();
  var date      = Utilities.formatDate(today, "EST", "MM/dd/yyyy hh:mm a")
  var col       = r.getColumn();
  var sheetname = ss.getName();

  r.setComment("Last modified: " + "(" + date + ")" );

}