code: func builtinTimeDiff(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	sc := ctx.GetSessionVars().StmtCtx
  	t1, err := convertDatumToTime(sc, args[0])
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  	t2, err := convertDatumToTime(sc, args[1])
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  
  	t := t1.Sub(&t2)
  	d.SetMysqlDuration(t)
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_timediff