I used to think season 3 takes place in 18 BBY, considering other media (like the Ahsoka novel or the Vader comics) take place in 18 BBY and the military transition from Republic-Empire has already fully happened, since we see Stormtroopers and Tie Fighters, for example.

But when you think about the passage of time during the show, it would technically place the final season in 17 BBY.

The show begins immediately after Order 66, which happens in 19 BBY. By that time, we would already be halfway through the year considering The Clone Wars seasons 5, 6, 7 and Revenge of the Sith all happen in 19 BBY.

A couple months pass during season 1, as confirmed by Senator Chuchi in season 2 when she says "funds were being diverted by Rampart months prior to the destruction of Tipoca City".

Then an unknown amount of months pass between the season 1 finale and season 2. During the Outpost episode, Mayday says he’s been assigned to protect the imperial cargo for "over a year". That indicates it’s not only been a year since the Empire began, but plus a couple months since the cargo was TK Trooper armor (which is where the Kamino's funds were going in the following months of the start of season 1). That would place that point in the season already halfway through 18 BBY.

And then, of course, there’s the 5 month time jump at the beginning of season 3. So does it make sense for it to be 17 BBY? If so, would that be a retcon?