I have always wanted to take a time lapse photo of New York at night, realizing there would be a problem dealing with the city lights with such a long exposure. Last night I gave it try while some clouds were moving behind the Empire State Building. I took the photo just after 4AM when the main lights that illuminate the top of the building were turned off.
I used the Fuji X-T1 with the Fuji 14mm lens. I calculated that I would need something like a six minute exposure to blur the clouds. So I put an 8x (3-stop) neutral density filter on the lens and set it for f/14 at an ISO of 200. I only had time for this one exposure because the clouds passed by and the sky cleared afterwards.
Long exposure (LE) noise reduction was turned on to fill in the tiny white dots that occur in the dark sky. With LE turned ON the camera takes a second exposure of equal length to the first one, but only of a dark frame. It then subtracts the dark frame from the original frame to eliminate the white noise specs. Even with the extra 6 minute dark frame subtraction, there were plenty of distracting specks left in the sky. I had to deal with them in Photoshop. To do this, I made a selection of just the sky, and added a slight, monochromatic Gaussian blur to it -- just enough (a 2 pixel radius) to meld the tiny white specs into the rest of the sky. The sky was already blurred from the long exposure so it didn't make much of a difference in the image sharpness.
To deal with the city lights, I processed the RAW file as a 16-bit tif and turned the highlights and whites all the way down in ACR (Adobe Camera RAW).