Hey Dmarley,
I'm typing this in Dutch, so press the translate button for the English version.
The first day is when the seed has germinated and the seedling shows its first 2 leaves.
After that, you just count the weeks.
And you start flowering when you see the first signs of flowering. For a regular plant, that's about 2 weeks after you set the lights to 12/12. And for autoflowers, it's about a month after they show their first leaves.
Good luck with your grow! 😋