1. A time management control method of a computer system for managing each individual time of a plurality of virtual systems, comprising: a step of acquiring a time of an overall system and a difference time between the time of the overall system and a time of the virtual system retained in a service processor, by means of firmware in the virtual system; according to a time change instruction, a step of calculating by the firmware a difference time between the time of the overall system and a change time of the virtual system, adding the both difference times, and informing the service processor; and a step of updating the difference times of the virtual system retained in the service processor, using the informed added times.