how to find date of manufacturer?

Try a google for "dji date of manufacture" it found what appears to be several useful threads.

It is printed on the box.

Hi there. Thank you for reaching out. According to India's Import Policy, Drones are 'prohibited' except for R&D and defense use. There are no DJI Authorized Retail Stores in India to sell drones. Simple consultations are available but aftersales repair service cannot be provided. Hope for your understanding and patience. Regarding the said request, we only can help to check the activation information of the drone according to the serial number. You may check the drone's activation date on the link below. Thank you.

https://service.dji.com/active/search