When the Galactic Empire was created at the end of the Clone Wars, the Star Destroyer design in service at the time was the Venator-class:
The Venator-class Star Destroyer was later phased out by the larger Imperial-class Star Destroyer:
In what year did the first Imperial-class Star Destroyer enter active service?
The canon novel Tarkin (which takes place 5 years after the creation of the Galactic Empire) provides a hint in that it mentions the existence and use of Imperial-class Star Destroyers:
Sentinel's Imperial-class Star Destroyer, the Core Envoy, and most of the flotilla's other capital ships were escorting supply convoys to Geonosis.
Tarkin, p. 16
We can therefore conclude that the Imperial Navy had already started using Imperial-class Star Destroyers within five years after the creation of the Galactic Empire. But when did the first Imperial-class Star Destroyer enter active service?
Canon sources are preferred; however, if canon sources are silent on the matter, I am interested in Legends sources provided such sources do not contradict the canon timeline.