Is the next deadline set from the day you turn in the last assignment or from the original deadline regardless?
Not sure if this is always the case, but I've always received an email from Glenda stating what my new deadline is. Generally it is from when your last assignment was turned in though. Say you turn it in a week before the original deadline, your new deadline would be off when you turned it in (or graded in some instances).
The new deadline is based on when the "trigger" exam (the last one assigned in the set) in the deadline you just completed is graded (whether or not you temporarily skipped anything before that exam), except for the very first deadline in a new module (which is based on when you get access to the module instead).
In Modules I and II, the grading date is the same thing as the submission date since the system will grade your exam automatically when you turn it in. In Modules III and IV, instructors or graders will be going over your work and grading it manually, so there may be a couple of days between when you submit it (meeting your deadline) and when it's graded (what the next deadline is based on). There is an additional explanation of how this works in the "Deadlines and Extensions" post in each module's Announcements forum, for more information.