A counter example to the rule on tense in relative clauses?

Question

In the comments to this question I learnt that

明日、林さんに会うときに、この本を返そう

and

明日、林さんに会ったときに、この本を返そう

have the same meaning, although 会った is more common.

With my current understanding, that the tense in a subordinate clause is relative to that of the main clause, I would have expected 会った to be the only correct choice, since I cannot return the book to 林さん before I've met up with him.

I was advised to post a separate question about this, so here it is. Could someone please explain how these sentences can have the same meaning and how it fits in with the relative tense rule.

Answer 1

Both are indeed correct, but I would understand とき slightly differently in those two sentences.

林さんに会ったとき refers to a specific point in time. The speaker will be with Hayashi-san then and he will return the book when that time has come. This is a typical example of relative tense.

I would understand 林さんに会うとき as referring more to an occasion than a specific point in time, or, in other words, a longer span of time than in the sentence with 会った. It sounds like the speaker will return the book taking advantage of the occasion to go out and meet Hayashi-san.

The probability of him returning the book to someone other than Hayashi-san seems to increase in this case. He might drop by a library on his way to the meeting, or meet the owner of the book after the meeting with Hayashi-san is over. He might as well return the book to Hayashi-san when the two meet, of course.

I think this can be seen as a case of absolute tense being used in a noun-modifying clause (recently discussed here), with とき being the modified noun.

Answer 2

I leave practically useful explanations to @aguijonazo's answer.

Linguistically this た does not indicate past tense, but perfect aspect. So this is outside any tense rule.

For example, this article cites the following:

松下（1930）は、「た」は動作性活用の語の第二活段へ附いて完了の意を表すものである。 現在、過去、未来、不拘時の何れにも用ゐられて其の完了を表す。

1 御覧なさい、綺麗な月が出ました。 現在の完了

2 私は子どもの時は国に居りました。 過去を完了に表す

3 借りたものは還さなければならない。 不拘時の事件の完了

4 明日伺ったらばお目に掛かれましょうか。 未来の事件の完了

The た in the question is the same as 4. The same article seems to discuss further how to sort out usage of た consistently, but for the current question, I suppose it is enough to note that た is not necessarily a tense marker.