Wikipedia notes in the page on April Fools' Day that
A study in the 1950s, by folklorists Iona and Peter Opie, found that in the UK, and in countries whose traditions derived from the UK, the joking ceased at midday. This continues to be the current practice with the holiday ceasing at noon, after which time it is no longer acceptable to play jokes. Ergo, a person playing a joke after midday is considered the "April fool" themselves.
"Countries whose traditions derived from the UK" surely includes the USA (which isn't mentioned separately anywhere in the page). Therefore, unless the Stack Exchange team are fools (and I don't think it would Be Nice™ to say that they are), the prank should only run from midnight to midday local time.