If you claimed it, then you can certainly still write it. That's what the amnesty period hopes to encourage.

The point of making a "period" for it is to just make it so people will all post their leftovers before other people start posting for the new round. It helps remove confusion about what's for the newest round. I won't complain if post at some other time, though. :3 Just make sure that you specify when the prompt was from when you post.