From Alex:
"The exactness of the calendar clock depends of the precision of the LPU board crystal. To avoid deviation, the calendar clock can be calibrated via command CATII . The calibration process is initiated when a BEGIN time is entered and terminated when an END time is entered. If there is a discrepancy between the system time and the END time then the real time clock is out of calibration. The system calculates a correction factor and adjusts the speed of the clock accordingly.
A longer calibration time gives a more accurate calibration. "
CATII:..........,CALIB=BEGIN; for starting calibration
CACAP; Prints out calibration data
CATII:..........,CALIB=END; for ending calibration