Subscription Nanodegree Programs:
Each of your projects will have suggested deadlines, but those deadlines are not required. The deadlines are intended to help keep you on track, to make sure you are successful in the program and reach your goals. You can always keep paying per month if you need more time to complete your Nanodegree program.
Term-based Nanodegree Programs:
If you are in a term-based Nanodegree program, there are two types of deadlines:
Ongoing suggested deadlines for individual projects.
The deadlines you see in your classroom are suggestions for when you should ideally pass each project. These "soft" deadlines are recommendations to help ensure you work at an appropriate pace in order to graduate on time.
A final term deadline for passing all projects in the Nanodegree program.
This is the date by which you must have submitted a passing version of every required project in your Nanodegree program. Passing a project in this context means that a Udacity reviewer has marked all of your required projects as “Meets Specifications” by the final term deadline. This final, absolute deadline is the last day on which you can still submit any projects. This deadline is enforced and can be found at the bottom of your classroom.
As long as you submit a passing version of every required project by your final term deadline, your project deadlines can be disregarded. It is very important to understand the distinction between the two, as project deadlines are optional (though recommended) while final term deadlines (sometimes also known as term end dates) are absolute and not optional.