Some strains might flower at 15 hours darkness. But is it really worth the risk.
Also the 12 hours of light is not derect sunlight. If the plants are in the shade but it's still sunny it's still part of the light period. Dark period is ttal darkness like night.
You should just be patient. BUt if you really need this done, cover it with a tarp ,leaving it in the light for only 12 hours. And kep the tarp on for the whole 12 hours of dark.
Some ppl here don't know about outdoor growing. Thats probably why they didn't say anything.
What strain are you growing???
It might flower it 15 hours but I doubt it.

Kush Over already told you all this.