Sometimes, this happens because of an error in our grading script. The operations team will quickly let us know if there's an issue. We'll also make sure to let the student know.
Please also note that all our assignment deadlines are listed in Pacific Time zone time, so your assignment may be marked late due to the current time zone you submitted in.
If you think your assignment was marked late in error, don't hesitate to get in touch with us at firstname.lastname@example.org.