How to calculate the 7-year life span of a medical device

Taking the date of equipment leaving the factory (or the date of acquisition) as the starting point, counting backward for 7 years. Within this period, medical devices can be used normally, but need to undergo strict overhaul and maintenance to ensure the normal operation and safety performance of the device. Some medical devices need to replace some key parts or components during the use process to ensure the performance and safety of the device, the time calculation method will need to be based on the actual use of the situation, rather than simply according to the fixed period of calculation.