I don't know why you would count anything other then completed time. Like was pointed out earlier in the thread you don't say a baby is 1 year old until it has completely the first year. The same goes for our babies in the garden.
They don't know what week or day you think it is
. They are just growing till they are done. Counting time greatly confuses the flowering process imho. Most new growers get these time frames in their heads and think the plants are going to grow according to what they are wanting. Then when they get to that arbitrary number they start freaking out "can I cut it, can I cut it"?
There are not many plants that will be finished and mature in 8 weeks of 12/12 time.
Most Indica dom plants will take 7-9 weeks of actual flower time. Most hybrid plants will take 9-12 weeks of actual flower time. Then hell Sativas can go anywhere from 12-16+ very easily.
I'm in no way saying that there aren't exceptions to this, but as a general rule these numbers are pretty safe averages. Cutting your plants early is the biggest reason for bad tasting or smelling buds, and is the main reason new growers aren't happy with their first harvest. You can't cut a plant early and then fix the smell with the cure. It might get better but it won't get as good as it would have been if you hadn't chopped early.