You can complete the course anytime and you codes will stay there in ide or with you.The deadline in gradebook is for students in Harvard and not for taking course on edx so you can pay and complete the course anytime by 31 Dec 2017 and you will be eligible for certification.
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You don't need an XID account for CS50x. You just need an Edx account to enroll in the course, which will also serve as your CS50 Submit authentication.
The only deadline for the 2015 offering of the course is December 31, 2015, by which all projects should be submitted. There is a proposed schedule for Edx students available if you need the psychological ...
From the Edx FAQs:
any scores you received on past years' problem sets will (soon) be
imported into CS50x 2017. (We’re still updating CS50x 2017’s
gradebook.) However, moving forward, you must submit CS50x 2017’s
problem sets, per the syllabus.
So from now on you can only submit via the 2017 gradebook.
However consider the two possible cases:
The 2016 course deadline has passed. The 2017 submissions haven't opened yet. But you don't have to resubmit any psets that have already been graded, as your gradebook will carry over to the 2017 course. You need only submit psets that you haven't already submitted. (but not yet, as the 2017 submissions system is changing and is not yet ready).
Starting now and trying to meet the deadline will be very difficult.
I am currently on week 7 (pset5) and it's going to be tight for me.
The deadline to submit all your problem sets(psets) and final project are due Dec 31 for everyone.
You can still do the course but you cannot get a certificate if you do not submit by Dec 31st.