The cortical array of a Borg drone
The chronometric node is a device implanted within the cortical array of every Borg drone. Its purpose is to record and contain physiological data about the individual drone, including the exact moment of death. (VOY: "Timeless")
In an alternate version of the year 2390, The Doctor was able to retrieve Seven of Nine's chronometric node which allowed him to pinpoint the precise moment when her cybernetic implants disengaged from her organic systems, thus providing him with her time of death: Stardate 52164.3, Borg time index 9.43852. This allowed Harry Kim and Chakotay to contact Seven in the past and prevent her death, as well as the destruction of the USS Voyager in the year 2375. (VOY: "Timeless")