When the seedling first pops out of the soil, I give it 1 week for what I call the seedling stage.
After that I consider my plants to be in the veg stage of growth and I start counting from there.
Usually 3 to 5 weeks of vegging and I trigger flowering.
At that point most of them are showing pre-flowers, so when I change to 12/12 that's my day 1 of flowering.
Good luck.