Go to the Effects Panel and open Video Effects > Color Correction; many of the filters there could do what you need. Something as simple as Brightness/Contrast may work, or you can get more advanced with RGB Curves or the Three-Way Color Corrector for more control. Experimentation is the best approach.
Along with Colin's suggestion, you might want to tweak the footage towards the blue end of the color spectrum. In addition, maybe add just a hint of Video Noise to help simulate the Day For Night. Another "trick," that might help create the illusion would be to apply Highlight & Shadow (be sure to uncheck Auto), and open up any Shadows a bit, as they will likely be too strong in the day footage. Your eyes, and some patience with experimenting on the overall look will go a long way towards success.
on another angle of approach... shooting day for night ...so it looks close to night or twighlight to begin with..before going to post with it...
check these links... notice the nd filter sometimes sells with the cool blue .. to basically knock your stop down as needed ( underexpose )..
main thing is you want high contrast and cool ( blue - ish ) colors ( to simulate twighlight or moon light )
excerpt from above link
Day-for-night (DFN) photography is an effect that makes a scene recorded in daylight to appear as if it were at twilight. This is usually accomplished by the use of a filter that both underexposes by about two stops, and can also produce a bluish color overcast. Lighting, contrast, and other factors contribute to the reality of this effect.
Tiffen offers the Cool DFN filter, which is a lavender-blue color traditionally associated with dusk or twilight emulation with day-for-night effects. The Tiffen Monochromatic DFN filter is a bright yellow-green, almost the color complement (opposite) of the Cool DFN. When you use this, and perform the necessary color-correction in post processing, you will not have all of the original colors available, producing a more realistic monochromatic effect that simulates actual silvery moonlight. Choice of which one to use is a matter of opinion. Both require an additional underexposure by ½ to 1-1/2 stops above not compensating for them in the first place. Advance testing is recommended.
samples....from some website
Jim, that tutorial is great !
Andrew's quite the tutor.