code: func (b *builtinAddTimeDateTimeNullSig) evalTime(row chunk.Row) (types.Time, bool, error) {
  	return types.ZeroDatetime, true, nil
  }
docs: // evalTime evals a builtinAddTimeDateTimeNullSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_addtime