Generally speaking, is the departure date the last working day or the next day?

It is not the last working day, nor the next day, but the date when the labor contract is terminated.

People's Republic of China (PRC) (China) Labor Contract Law

Article 37? The employee may terminate the labor contract by giving a written notice to the employer 30 days in advance. The employee may terminate the labor contract by notifying the employer three days in advance during the probation period.

Regulations on the Implementation of the Labor Contract Law of the People's Republic of China

Article 24 The certificate of dissolution or termination of the labor contract issued by the employing unit shall specify the term of the labor contract, the date of dissolution or termination of the labor contract, the post and the working years in the unit.

Delayed information resignation procedures need to be handled according to law:

People's Republic of China (PRC) (China) Labor Contract Law

Article 50? The employing unit shall issue a certificate of dissolution or termination of the labor contract at the time of dissolution or termination, and go through the formalities for the transfer of files and social insurance relations for the workers within 15 days.

Laborers shall handle the work handover according to the agreement of both parties. If the employing unit should pay economic compensation to the workers in accordance with the relevant provisions of this law, it should pay it when the work handover is completed.

The employing unit shall keep the text of the dissolved or terminated labor contract for at least two years for future reference.

National People's Congress-People's Republic of China (PRC) (China) Labor Contract Law

National People's Congress-Regulations for the Implementation of the Labor Contract Law of the People's Republic of China