On September 28, Mega Gyarados Raid Day was announced for Saturday, October 8 with an advertised start of 2:00 p.m. local time. However, it seems that the plans have changed a bit according to Reddit user FinchyNZ:
This time-change combined with the lessening of Mega Energy may throw a wrench in the plans for many players, and we have no idea if it will change as more and more timezones roll over to 11:00 a.m., so be prepared to change your plans accordingly!
Update From Niantic
Niantic has acknowledged this mistake, and has officially stated that the new global time for Mega Gyarados Raid Day is now 11:00 a.m. till 5:00 p.m.
This gives us three additional hours to take advantage of this event.
Trainers, we're aware Mega Gyarados Raid Day started at 11:00 a.m. local time rather than the announced 2:00 p.m. in earlier regions. As such, we have decided to extend Raid Day in all regions from 11:00 a.m. to 5:00 p.m. for a total of 6 hours instead of the previous 3 hours.
— Niantic Support (@NianticHelp) October 7, 2022