So in the example below the Shinano started to be built when its delay was 1800. The value in Delay is reduced by 180 each turn, while start building increase, al-be-it negatively, by 180 each turn. The two should add up to 1800.
No the delay(days) is reduced by 1 day each turn, unless you accelerate the vessel.
So next turn Delay will say 1001 .. And "WHEN IT .." Started Building will increase to -799.
So at present it is showing the CV will arrive in 1002 Days.
Durability and build cost are the same when you are building normally.
If you accelerate, the durability stays the same, but build cost = durability * 3 (540) and the accelerated delay, the delay will be halved to 501 Days.
Is it starting to make sense ?