How you think marijuana grew before people came up with indoor growing tecniques?!?
The switch from 18/6 to 12/12 is just done to replicate the changing of seasons from summer to autumn. It's too late to start now but basically you want to plant in early May, then it will naturally go through fast vegetative stage during long summer hours, then, as autumn comes, your plants will notice the shortening of daylight and begin the flowering stage. So nature does the work for you!!
I'm currently growing in Ireland and as the summer comes to an end, my plants have just begun to flower.
I'm sorry to say I think you're going to have to wait till next year if you want to grow outdoors.