The 59309A HP-IB Digital Clock is just that, a digital clock. The date and time can be both read and set via HP-IB.
Today, engineers would typically drive automated test instruments from a computer that has real-time clock capability, but that generally wasn't the case in the 1970s. The 59309A allowed real-time clock functionality to be embedded with the test equipment. A controlling computer system would be able to poll other instruments for their measurements and time-stamp them with the date and time received from the 59309A, or poll the 59309A and issue commands to other test equipment based on the date and time.
The instrument has a rear-panel BNC connector for an external 10MHz frequency reference, as well as external standby power. The related K10-59992 instrument is a battery box in the same enclosure style and size as the 59309A, intended to be mounted with it in a cabinet.