1. A time management method of a computer system for managing each individual time of a plurality of virtual systems, the time management method comprising: acquiring a time of an overall computer system in a time management hardware of a service processor connected to the plurality of virtual system and a first difference time between the time of the overall system and a time of the virtual system retained in a memory of the service processor, by firmware in the virtual system, to set the individual time when starting the virtual system; calculating by the firmware a second difference time between a present time of the virtual system and a change time of the virtual system according to a time change instruction of the virtual system; adding the second difference times to the first difference time; informing added times to the service processor; and updating the first difference time of the virtual system retained in the memory of the service processor, using the informed the added time.