Step-by-step explanation:
Well what I would think is that after the 31st of whatever month we're referring to, they may unlock the post test or something similar which may kinda affect your grade. It's supposed to be self - paced since no one's gonna rush you into doing it unless you have a teacher making you or something.
Edit: Forgot to mention, if it says like a deadline by December or whatever, either it will just continue with your previous units or it will automatically start you with whatever units you'd have to do to finish the school year.
For example, 1st semester we get units 1-3, next semester might either be 1-6 or 4-6. Maybe not even 6 units but more or less. And even if you fail one part, it would still count it as you have done it, so you won't have to keep trying unless your instructor urges you to keep trying until you pass.