Last day to deposit employer contribution to individual 401K

Can you please confirm what is the last day to deposit employer contribution to individual 401K ( which is maxed at 25% of payroll amount for the year ) for FY2020? I understand the last day to contribute the employee part is Dec 31 . 2020. Is that correct?

Typically the individual 401 K plan provider does not distinguish between employee and employer contributions and I am concerned that if I deposit the employer part in Jan 2021 will that create any confusion?

Thanks