A time index identifies the temporal coordinate of a piece of data in a data stream or data clip.
In 2370, Keiko O'Brien noticed an anomaly in the data clip of the security record of the death of her husband that eventually led to the discovery that the clip was a forgery. The time index showed that it was afternoon, and her husband was drinking coffee, which she believed was uncharacteristic. (DS9: "Armageddon Game")