You mean, if you had light for 12 and darkness for 15?. Bad idea, you have to keep within the natural 24 hour cycles. (12 + 15 = 27. No, no, no) But if you mean, darkness for 15 hours and light for 9, Yes. The plant thinks that winter is getting ever closer and she will flower faster. If you are wanting faster flowering, without wasting too much bud, try 11 hours light, 13 hours dark.