It is not correct to say that the tense of the verb in a reported time clause never changes. It depends on whether the action or state in the time-clause is still true at the time of reporting.
As an example of no tense change, imagine that Person A, three months ago, said to person B: "I will go to Italy when I finish school".
If Person A is still at school, then Person B will typically report this to Person C as:
- She said she'll go to Italy when she finishes school.
If, however, Person A left school one month ago, then Person B would most likely report the statement to Person C as:
- She said she would go to Italy when she finished school.
If Person B used the present tense here, it would imply that Person A is still at school.
The same principle applies in other tenses. Example: Person A says to John: "I'll call you back as soon as I've spoken to Maria."
If Person B knows that Person A has not yet spoken to John, then he or she would typically report this to person C as:
- She said she will call John back as soon as she has spoken to Maria.
If, on the other hand, Person B knows that Person A has now spoken to John, then he or she would report this to person C as:
- She said she would call John back as soon as she had spoken to Maria.
backshifting the tense in the time clause from the present to the past perfect.
So, in summary, it is not true to say that tenses in time clauses never back-shift in reported speech. It depends on how the reporter wishes to convey the current validity of what was said to him or her.