Do they keep you employed for that month? It's fairly common to extend your termination date for health insurance and visa reasons. If you aren't actually terminated, the 90 days wouldn't have started.
The sentence was slightly hard to parse so you may have missed that you get an extra month for each month you are employed over 1 year. So the stated example of a 2 year tenure gives 1 year and 90 days to purchase the options.