In the period of national planned economy, people who work and study part-time are not official employees of the state, and they are not employed by the labor department, and their length of service is not counted. After the implementation of the labor contract system, the payment period is calculated based on the payment of endowment insurance, and the pension is calculated according to the payment period. It is useless for the unit to admit the length of service.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.