If you apply for a trademark in Beijing by yourself, the notice will be mailed to the address you used when you applied. If it is through an agency, it will be mailed to the agency first and then to you. These 30 days are the date when the notice arrives at the agency and the materials are sent out. If the date at the post office is unclear, the date it is received by the Trademark Office will be used as the date. The agency will ask the post office to stamp it, so the time is generally not unclear.