System for synchronizing logical clock in logical partition of host processor with external time source by combining clock adjustment value with specific value of partition

An external time source is connected to a partitioned data processing system, having host processors controlled by a host hypervisor, and having operating systems in the partitions. The host processors each have a timer facility comprising a time-of-day (TOD) clock, and a clock comparator. When the hypervisor detects a need for synchronization between the external time source and a host timer facility, it insulates the operating system in the partition on that host from host synchronization, and synchronizes the host timer facility with the external time source. Subsequently, the operating system is placed into normal execution, with an adjustment value used for timer facility references, and with a synchronization interrupt pending if the operating system is aware of the external time source.