The date of release on the S&S website is the date of release of the eBook. That is why I chose that date because it's a fixed date and everyone then has a chance to have at least the same amount of time to read.
I understand why you chose the date. That's not the point. You're not the one who makes the rules around here. The mods do that. The six-month mark for each book is whatever they say it is.
Given that the date on the S&S website is the official date, I don't see any reason not to use that date. Do you want to risk the wrath of the mods because you don't give a darn?