Usually the idea would to start timing from the 12/12 simple because the plant changes after this. When you turn off the lights for 12 hours it induces a biochemical reaction in the plant that says hey baby it's time to flower. The breeders times are usually 1-2 weeks shorter then they actually need because people like faster flowering strains and they tend to sell better. But for everyone reading and to try to have some commonality amongst growers.. The switch to 12/12 is when the vast majority of growers and banks start counting. Why? Well because lots of strains flower at vastly different times.. The vast majority of folks on here are growing hybrids that don't have the really long flower times like some Landrace Sativas, etc. have.