We had long delay because doors were mismeasured and then firm didn't complete finishing touches because we enfoeced late penalties, including leaving open permits. Post if you'd like to discuss fully.

Is there a reason why the 12th floor residents can't take the elevator to/from the 11th floor and walk up/down 1 flight of stairs? Is there a problem with the 12th floor - the door doesn't work or doesn't exist?
As Carin stated, having a firm deadline and penalty clauses may reduce the timeframe. I would be flexible on the start date until the required parts are on site and checked to see that they are to specs. Push back the start date if necessary until everything has been checked.
I was recently told by our elevator company that there are new NYC elevator requirements that need to be in place by Jan 1, 2020 that will take most NYC elevators out of service for the work required. He also recommended upgrade of our control panel due to its age (only 18 years old) as parts are getting harder to obtain when needed. With the control panel replacement, the elevator could be out of service for 4-6 weeks. Ballpark cost per elevator with the new control panel $80K.