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Is there really a requirement to apply 30 days before?
Not that I am aware of, but...who am I

Generally and, unless things have changed, a person can apply to extend their status one day before their current [valid] status expires. This is when a person's status changes to `Maintained Status' while IRCC reviews the application.

However, if the application is returned (or not received by IRCC), the person is NOT covered under maintained Status, but would have 90 days to then apply for Restoration of Status. Unfortunately the person would be without legal status while the ROS is reviewed/processed.