Internal chronometer

From Memory Alpha, the free Star Trek reference

The internal chronometer was a part of a Soong-type android's construction. It functioned as a timekeeping device and can be switched off. (TNG: "Timescape")

Data's internal chronometer was stopped by the solanogen-based lifeforms when they abducted the android. When comparing his chronometer to the USS Enterprise-D's chronometer, Data realized that he had been off the ship for more than 90 minutes. (TNG: "Schisms")

In 2369 after Counselor Deanna Troi saw Captain Picard, Data, and Geordi La Forge "frozen in time", Data checked his internal chronometer but couldn't find any discrepancy between his chronometer and the ships computer. (TNG: "Timescape")