External Time Reference hardware facility (ETR) is the generic name for IBM Sysplex Timer.
The ETR architecture provides the means of synchronizing TOD clocks in different CPCs with a centralized time reference, which in turn can be set accurately on the basis of UTC time standard (External Time Source). The architecture defines a time-signal protocol and a distribution network (called the ETR network) that permits accurate setting, maintenance, and consistency of TOD clocks.
CPC – Central Processor Complex
TOD – Time of Day
UTC – Coordinated Universal Time