code: func (b *builtinUTCTimeWithoutArgSig) evalDuration(row chunk.Row) (types.Duration, bool, error) {
  	var nowTs = &b.ctx.GetSessionVars().StmtCtx.NowTs
  	if nowTs.Equal(time.Time{}) {
  		*nowTs = time.Now()
  	}
  	v, err := types.ParseDuration(b.ctx.GetSessionVars().StmtCtx, nowTs.UTC().Format(types.TimeFormat), 0)
  	return v, false, err
  }
docs: // evalDuration evals a builtinUTCTimeWithoutArgSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_utc-time