So as of recently I've notice a slight change in building time. Like a lv13 diner that says 21hrs to complete but once started it says 17hr 59min left.... I've got 2 builders. Perhaps there is a bonus to having more than one worker? Or is it a bug and honestly I don't mind.... but wanted to bring attention to it.
Accelerated time table
I would have to say they changed it without changing the posted times. As your buildings gain levels the time difference widens. I cannot say for sure it is not a bonus for # of builders (I have 3). But it does not seem like it.