Hello, @mattbrenda.
Thanks for reaching out to the Community.
I'll be happy to help with updating the paydate on a check.
A paydate of 12/31 would apply for 2018 and for the 4th quarter. Depending on which Payroll service you're using, your tax forms may have already been completed and filed. If you need this payroll to apply to q1 of 2019, you'll want to contact support to have a corrections case created and possibly even amendments.
You can contact support, here.
For more information on Payroll corrections, click here.
Please remember, I'm always here for any other questions or concerns.