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A year was the amount of time it took for a planet to orbit its sun. If the planet had an axial tilt, this resulted in seasons, like winter, spring, summer, and fall.

Vulcan year: In Gene Roddenberry's novelization of Star Trek: The Motion Picture he indicated that nine Vulcan seasons were equal to 2.8 Earth years. This would make Vulcan's year 456 ± 33 Earth days long.
The term "turn", used in the episode "Sins of the Father", was, according to the Star Trek Encyclopedia (3rd ed., p. 528), "the English translation of the Klingon term for year, as in one turn (or revolution) of a planet around its sun."

It took the colony ship SS Conestoga nine years to get to the planet Terra Nova to form the Terra Nova colony in 2078. (ENT: "Terra Nova")

Sub-commander T'Pol stated in 2151, that she was not familiar with the early years of Earth space exploration. (ENT: "Terra Nova")

The Vulcan monastery at P'Jem was almost 3,000 years old as of 2151. According to the Vulcan elder at the monastery, the reliquary wasn't entered in years and the transmitter also hasn't been used for years. (ENT: "The Andorian Incident")

With a warp three engine, a cargo freighter could cut down a five year cargo run down to six months. (ENT: "Fortunate Son")

Nausicaan pirates were attacking freighters in a specific sector for years until 2151. (ENT: "Fortunate Son")

Standard UFP solar year

Standard UFP solar years were mentioned in the Treaty of Armens. (TNG: "The Ensigns of Command", okudagram)

Earth year

One Earth year was a span of time equal to 365.2425 Earth days.

Captain Jonathan Archer suggested that Bernadette Fuller was around seventy-five Earth years in 2151 while talking to Nadet. (ENT: "Terra Nova")

According to Doctor Phlox, Travis Mayweather was twenty-six Earth years during his autopsy in 2152. (ENT: "Dead Stop")

There was a reference to "terrestrial years" in dialogue in the revised final draft script of TOS: "The Conscience of the King". In the final version of that episode, however, this reference doesn't include the term "terrestrial".

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