According to the US National Institute of Standards and Technology, December 31 will last this year in parts of the world west of the Greenwich Mean Time meridian not just 24 hours but 24 hours and one second.
In the eastern United States, the addition will occur at 7:00 pm and should have no significant effect other than to stretch out by a second the December 31 happy hour. [I assume that means its at 6pm central time]