code: func strDatetimeSubDuration(sc *stmtctx.StatementContext, d string, arg1 types.Duration) (string, error) {
  	arg0, err := types.ParseTime(sc, d, mysql.TypeDatetime, types.MaxFsp)
  	if err != nil {
  		return "", err
  	}
  	arg1time, err := arg1.ConvertToTime(sc, uint8(types.GetFsp(arg1.String())))
  	if err != nil {
  		return "", err
  	}
  	tmpDuration := arg0.Sub(sc, &arg1time)
  	fsp := types.MaxFsp
  	if tmpDuration.MicroSecond() == 0 {
  		fsp = types.MinFsp
  	}
  	resultDuration, err := tmpDuration.ConvertToTime(sc, mysql.TypeDatetime)
  	if err != nil {
  		return "", err
  	}
  	resultDuration.Fsp = fsp
  	return resultDuration.String(), nil
  }
docs: // strDatetimeSubDuration subtracts duration from datetime string, returns a string value.