These flower times are more what they can be under perfect conditions. More of a guideline kind of thing then a exact finish date. Things like from seed or clone effect that. If your indoors how long you vegged them, the amount and quality of the light all can have some effect on finish times.Non organic, heavy feeders may need a little longer of a flush. The most accurite way to know when to start flushing is by looking at your tri-comb coloring. If you take your flowers to 50/50 amber then when they are mostly all cloudy you can flush for a week and some change while they darken up. If you like more couch lock highs then let it go longer, it does reach a degrading point but in general mostly amber is more couchlock-ish and when harvested early more cloudy its more energentic. Those things will also have an effect on the date you choose to start flushing.
The free grow guide ILGM offers talks about when to harvest and its good info and easy to understand. Fyi