Chronometric node

From Memory Alpha, the free Star Trek reference

The chronometric node is a part of the cortical array implanted in every Borg drone, which measures the passage of time. (VOY: "Timeless")

In an alternate version of the year 2390, The Doctor retrieved Seven of Nine's chronometric node and determined the precise moment when her cybernetic implants disengaged from her organic systems, i.e. 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")