Yes, you can defer a class date - once.

About 4 weeks after you get the CJO, they email you a class date, and tell you that you must be available full time for 12 weeks starting on that date.

if you need to start a few weeks or months later, just reply to the class date email, politely explaining that in order to make the 12 week commitment, you will need a later start date. Tell them the date when you’ll be available.

They don’t guarantee a specific start date of your choice, but will email you about 4 weeks before the date you said you will be available, giving you your new class date.

I don’t know what the max delay is that they would accept. In my case I needed to shift start date about 6 weeks in order to not miss a big family event. They assigned me the first Tuesday after my available date.