Temporal field generators were a technology able to create a form of energy capable of allowing for time travel.
Such a field generator was an integral component of 29th century timeships. Upon activation of the field generator, it was possible to create a temporal rift, but the stability of the temporal field created was determined by the proper calibration of the ship's temporal matrix. In an alternate timeline in 1996, 20th century industrialist Henry Starling utilized the stolen timeship Aeon to travel into the future, but the instability of the temporal field he generated caused a massive temporal explosion in the 29th century, destroying all of Earth's solar system. This timeline was later negated by the crew of the USS Voyager, who were able to destroy the Aeon moments before it entered the rift. (VOY: "Future's End", "Future's End, Part II")
A time portal discovered on the Bajoran colony world of Golana utilized temporal field generators. In 2374, after eight-year-old Molly O'Brien accidentally fell through the portal, arriving three hundred years in the past, her father, Miles O'Brien, Deep Space 9's chief of operations, was successfully able to modulate the portal's generators to reactivate it and retrieve Molly with a transporter beam. (DS9: "Time's Orphan")