day 1 is the first day it pops out of the soil. if you're talking flowering... day 1 for most is the day you switch the lights to 12/12 (even though the plant isnt actually flowering yet).
I call that day one of transition. First day flower (for me and many others) is when I see the flowers.
you are right by going what the breeder says; the seed breeder I use says upon the appearance of flower.
How I do schedule Germination first day I put in water or towl to germ until it pops a tap root (some times this can take days) Then you restart from day 1 for the next stage in growth So once the seed pops out of the ground that starts day 1 until it has sprouted 2 sets of of true cannabis leafs then that stage ends Veg starts Then flower starts ( usually you would veg for like 2 months before flowering if you want any type of yield) MPM