Day Counting.....

Who really cares. She is rdy when she is rdy, it's only supposed to be a guide, not a definitive number. It helps if you yourself can be consistent in your methods across grows so you yourself can better judge how well your doing to get plants through the seedling stage to see if you can improve upon the timings etc. I wouldn't put too much thought into it unless your particular about details, just pick whay works for you like everyone else.

When its showing. Like a Birthday, I count when the seedling sees the sun.

I count the start of the seedling/vegetative phase from the day the cotyledons have fully opened and lifted. For me, the first day of flowering is the day I switch the lights to a 12/12 schedule. That’s how I approach my grow.

do what's best for how you want to use that information. I track days to sprout on it's own so i can re-assess my method of sowing seeds. When the bulk don't sprout within 2-3 days i know i did something wrong or have some old seed etc. I count 'vegetative' days the moment it breaks ground. This allows comparison of growth in a more apples to apples way between growing plants. Again, helps re-assess methods or any adjustments made and compare to a baseline/expectation. As far as flower phase, whehther it technically flips (unseen biological processes initially) 24h or 72h after changing light schedule is irrelevant. I just count from light schedule change. if it were an autoflower, i'd go by ~7 days before i saw the first preflowers at nodes. I don't grow autoflowers, so not an issue either way, lol. When you see it is not when the plant went into flower. Flower phase takes just as long and just as variable in autoflowers. I also count days/weeks etc after the day finishes not while they are ongoing. Small detail, but can be why my "week 6" is more like a "week 7" in someone else's diary. When i add extra columns like wattage used to spreadsheet, it makes the formula easier if i don't have to do 'n-1' or whatever, lol. Otherwise it'd be including an extra day in the calculation and give a false estimate of kw-h used. Just an example of how you count can be dictated by other things and not a recommendation. As long as it works for you, i wouldn't worry about much else.

I count from when the seedling is above ground and the cotyledons open to reveal the first pair of true leaves. Before that stage, the plant is not a functioning being in my opinion.