The completion date on your certificate is generated by Greenlight Locker. It can be different from the date you completed your work.
This can happen because of the way dates are handled by computers and servers. It is most common if you complete your work late in the evening.
Certificate times in Greenlight Locker are based on UT (Universal Time, a.k.a. GMT/Greenwich Mean Time) and that is several hours ahead of anyone in the US, depending on time zone. So, a late evening completion in your time zone could result in a "next day" completion date in Greenlight Locker.
This can also occur if your certificate processing is delayed for some reason, such as having completed your certificates in more than one account. Manual processing can result in a later issue date on your certificate.
If there is any concern about your assignment being late, teachers can review the Resources Studied for study session times in their local time zone, rather than the UT time at which the certificate was issued. Teachers can reach out to the TCB/EdReady team for help if they have any questions, using the HELP button at the bottom of the screen.