[KyleGoetz]

Originally Posted by ByTheWay

Ok, I think i've got it. It's a bit like - 'by next week I will have typed the report'. Like a past participle to indicate an action that will be completed at some time in the future, hence the reason why it does not have a past tense.

今時間がないので来週会ったとき話しましょう

means, I don't have time right now, but by next week we will have met and conversed.

Basically, 会った is modifying とき. とき="time" so it's saying, in effect, "the time at which we met already," or something to that effect. However, you don't want to use such a literal translation.

Here, it's saying something like "let's talk at the time-we-have-met."

Another example:
日本に住んでいるとき、日本語を勉強していました。
(I-am-)living-in-Japan time, I studied Japanese.

More artfully,
When I lived in Japan, I was studying Japanese.

Beginners and intermediates alike make this mistake in constructing the sentence:
日本に住んでいたとき、日本語を勉強していました。

Another example:
日本に行くときに、日本語を勉強したほうがいいよ。
go-to-Japan time, you should study Japanese.
日本に行ったときに、日本語を勉強したほうがいいよ。
went-to-Japan time, you should study Japanese.

The first sounds like before you go, you should study. The second sounds like once you've gone/while you are there.

At least, that's the way I think about them. I know even people at my lower-advanced level still make that mistake—using verb modifier + とき is a tricky thing! I may have made a mistake; if so, I hope I get corrected.