During the Developer Update Livestream, images have been posted that included a countdown of the time when Diablo IV Season 1 will end.
While we can currently not tell if those were just placeholders of be accurate dates that have been leaked, we can still speculate that this time might be accurate.
Diablo IV Seasons are scheduled to be released every 12 weeks, which would lead to Season 1 to end just a few days earlier then that. This might actually indicate that these dates shown are correct, but no official statement has been given by Blizzard yet.