you cant force an outdoor plant to flower, if its in a pot then you could bring it in to make sure its getting 12 hours of light followed by 12 hours of dark, the plant will then flower, but they take around 2 weeks to start flowering from when the light schedule is right, so most outdoor plants will be starting to flower now if their not flowering all ready, it also depends on age of plant as young plants need to mature before they start flowering, so this is around 6 to 8 weeks from seed,
plants flower under 12-12, so anything around that is good but 12-12 is best, basically the dark period needs to be equal or longer than the day period, indoors we use 12-12 when we want to flower, outdoors nature does it for us when the sunset and sunrise is close to 12-12,
their is no way of speeding it up, you can make sure its getting 12-12 by bringing it inside if its in a pot but if its in the ground then you dont have much control, some growers cover their plants so they can flower them before the schedule is right, check what schedule the light is on for where you are, use search engine and check for sunset and sunrise times, plants start flowering around 13 hours of light and 11 hours of dark, this is when flowering usually starts and then it continues as the days get shorter and nights get longer
Meet the Opaque coche.
These things are Old Skool. gardeners have been using them to force early flowering for hundreds of years, just emulate them as best you can for a couple of hours, just until its dark enough at night to remove them. =)
interesting idea GiGa, i did come across a grower a while back who was using big paper type bags, his plants where 5ft plus high so he made these big paper bags and used them to cover the plants to induce flowering, as long as you set an alarm and remove the cover at the same time each day then it wont cause any problems, but messing the light schedule up will cause the plant to go hermie, so it is possible to flower early and im sure it wouldnt take much to make something to cover the plants, if i was growing outdoors then id flower during the middle of summer, id start plants indoors in big pots then move them outdoors for the 12 hours before it gets dark then bring them back in for the 12 hours of dark, but i cant grow outdoors so indoors it is.