Temporal When (Verb Tense Variants)

Where the action of the とき clause sits relative to the main clause.

Verb るときに vs Verb た ときに: This is tense inside the clause, not register. るとき means the action has not yet completed at that moment (日本へ行くときに切符を買う: bought before going), and たとき means it has (日本へ行ったときに買う: bought after arriving). The main verb's tense has no say in it.