Legality is determined at the release of a Standard-legal set on Magic Online.
Prices are checked every hour for a week. Anything 1c or less for half or more of all checks is legal for the season.
Cards from the just-released set are added (nothing removed) a couple of weeks later via a supplemental rotation after prices have settled a little.
Any version of a card on the legal cards list is legal.
The supplemental rotation is in 13 hours, 39 minutes (The next full rotation is in 7 weeks, 1 day).