The Intrepid, first ship of its class, entered service around the year 2153. (ENT - Rise of the Federation novel: A Choice of Futures) The ST reference: The Official Starships Collection states the Intrepid class predated the NX class. In April 2153, the Intrepid and two other starships came to the rescue of the Enterprise (NX-01) when it was attacked by a Klingon Bird-of-Prey as it entered the Sol system. (ENT episode & novelization: The Expanse)