While I can't agree with your advice to remove lots of leaves. I do agree with this about flower days.
I neglected to post about that in my first comment and had ment to. It looks like this plant is just now starting to push bud sites out. So this plant is just now starting to flower. I would probably even wait a couple more days before I would call it started honestly, but thats niether here or there.
Flowering begins when the plant starts to form flowers, not instantly at 12/12 flip. It takes time for plants to transition from veg into flower mode. In your case it seems to have taken a little longer, and I would guess that it is because you only vegged for 21 days from seed. Most plants take between 4-8 weeks of veg time to be mature enough to start flowering. You can switch the lights whenever you want, but the plant won't start flowering until its mature enough. Thats why if you grow a plant under 12/12 from seed that it still takes several weeks to begin flowering.
If you had a plant outside you wouldn't say it was flowering until it started forming flowers. You wouldn't pick an arbitrary number and decide it instantly started flowering on that day. You would observe the plant and watch the signs it gives you.