Hey Philippe,
First, make sure you're using the latest version of TurboActivate. If you're not, then try that first.
Next the following three things must be correct:
- The timezone must be correct. (This should be checked and fixed before anything else).
- The date must be correct.
- The time must be correct.
My guess: the customer is using a version of your app that uses an old version of TurboActivate or their timezone is wrong. Those are the most common mistakes.