The arrival of 1991 has been postponed.
The new year will be a second late, to accommodate a "leap second" that will be inserted Dec. 31 to keep clocks in time with the Earth. While modern atomic clocks are accurate to within a billionth of a second a day, the rotation of the Earth isn't nearly that consistent. It speeds up a bit sometimes, slows down other times.
The leap second will occur at 23:59:59 universal coordinated time, formerly known as Greenwich Mean Time. That's 6:59 p.m. and 59 seconds, Eastern Standard Time.
Normally, 6:59:59 p.m. EST is followed by 7:00:00. But on Dec. 31 of this year it will be followed by 6:59:60, and that will be followed by 7 p.m.