In what year did the first Imperial-class Star Destroyers enter service?

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.

Answer

Short Answer

19 BBY

Long Answer

During the Clone Wars, there were at least four Star Destroyer classes in use:

"I hate the look of these new mass-produced Imperator-class Destroyers," Gayn continued. "None of the artistry that went into the old Acclamators and Venators—even the Victory Twos." He shook his head in disappointment. "So goes elegance."

Incept-dates of 22 BBY

For the Venator and Tector class ships, Wookieepedia lists 22 BBY, and as far as I know, it is not fully settled what the full 'Navy' complement and organisation was, prior to the Clone Wars and the receipt of the Clone Army, except that:

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  a) The Republic did have ships prior to the Clone Wars which was some combination of Jedi Forces, Judicial Forces and the Planetary Security Forces, but its exact size and composition is not clearly/fully stated, and

  
  



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  b) When the Republic received the Clones from the Kaminoans, they also suddenly had enough ships to transport them in, and conduct war operations with

  
  

The Tector-class ships only came into being at the beginning of the Clone Wars, but whether or not any Venators existed prior to 22 BBY is not clearly stated as far as I can find.

The general issue is discussed somewhat here, and the discussion on this page suggests that the Kaminoans ordered the ships at the same time as they were producing the Clones, which would confirm an incept-date of at least 22 BBY for the Venator and Tector classes.

Notes

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  In the above quote from the Luceno book, the character Brudi Gayn references the Victory-Twos which may have been introduced after the events in the book, but this may just be an error on the part of the author.

  
  
  


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  In Star Wars: Dark Lord The Rise of Darth Vader by James Luceno, the Executrix is also mentioned as the older sibling of the Exactor, which is more firmly established as an Imperial-Class I Star Destroyer (as it was the flagship of Vader immediately following the end of the Clone Wars), lending credence to the position that the Executrix is an Imperial-Class I Star Destroyer as well, both of which existed in 19 BBY:

  
  

The Star Destroyer Exactor and its older sibling, Executrix, drifted side by side, bow-to-stern, forming a parallelogram of armor and armament.