"DAY 1" of flowering - exactly when?

Say that when you go to flowering, it's a week-long transition of an hour a day.

When exactly is "day 1" of flowering?

• The first day you cut back from 18/18?
• The first day you start 12/12?
• Or the first day you see signs of actual flowering?

It's usually counted from the first onset of flowers. The reason behind it being different length of the period between switching to 12/12 and the first flowers. This way you ALWAYS know what eg. 5th day of flowering means.

Well, it doesn't really matter that much, because flowering times aren't exact anyway. If a strain is listed as being an 8-week strain, you can really plan on them to be finished at exactly 8 weeks. As long as you have a rough idea when you started 12/12, you can keep an eye on them as they mature, and have a good estimate of when they will finish. Ultimatelly, you'll have to watch the trich and pistils to know when they are done.