legal ground
Article 28 of the Patent Law of People's Republic of China (PRC),
The date when the patent administrative department of the State Council receives the patent application documents is the filing date. If the application documents are mailed, the postmark date of mailing shall be the application date.
Article 30
Where an applicant claims the priority of a patent for invention or utility model, he shall make a written statement at the time of application and submit a copy of the first patent application document within 16 months from the date of the first application.
Where an applicant claims the priority of a design patent, he shall make a written statement at the time of application and submit a copy of the first patent application document within three months.
Where the applicant fails to submit a written statement or a copy of the patent application documents within the time limit, it shall be deemed that the priority has not been claimed.