you shouldn't cut the dark time. If you want to shorten though you can do less light hours, but you will need a digital timer to set up a good schedule, or just build your entire life schedule around turning your lights on and off. Not sure what this does to your yields as I have never tried it myself.
i remember an article in Red Eye Magazine which reckoned you can get shorter flowering times by making 13 dark/11 light, but you get less weed. The conclusion was do what suits ya, commercial or sweet sweet bud