Who counts like that? I've never met a grower that counted like that. That would mean my plants are 2 weeks in looking like this?
Most real growers understand how the plant biology works, and don't count from an arbitrary point like the 12/12 flip. It takes time for plants to transition into flower from veg. Trying to include the transition period into the flowering time is one of the biggest reasons new growers especially try to harvest plants too early.
You certainly wouldn't say a plant is flowering outside until it started stacking nodes and forming flowers...... Well the plants biology is the same inside, you aren't a god that controls its every action. You can direct the plant by switching the lights, but it must be genetically mature enough to flower before it will. Have you ever ran plants 12/12 from seed. It still takes weeks before they start to flower because they must be mature. Along with being mature, the specific genetics and the environment will play huge factors into how long it takes for plants to actually begin to flower after the flip.
Counting from the point the plant actually starts forming flowers puts you on its genetic timeline. Which gives you the best ability to grow the plant to full potential.
That being said I don't count weeks anymore. I havn't for years. There is very little point. Once you watch the plants and learn how they grow you treat them appropriately. They get harvested when they are done, not based off a number of weeks.
Plants grow differently in different environments as well, and it might take a crop of identical clones different time frames because of one variable or another.