That is correct. Your FIRST extension date counts as day 1.
So you obtained extension Sept 29.
Your starting date is September 29, 2024 so that means that 89 days later would be December 27, 2024.
Attend CW prior to that date.
Perhaps there are no appointments available prior to that date.
You can report in person up to one week late and not receive a fine.
Fine is 2000b