A minute is always sixty seconds long

[ThatJoeMoore]

A UTC minute can be 59, 60, or 61 seconds long, due to "leap seconds." Leap seconds are added (or subtracted) at the end of a month to account for changes in the rotation of the earth, such as those caused by the moon, earthquakes, and man-made events. As of 2017-03-24, 27 leap seconds have been added, and no negative leap seconds have been applied.

When a leap second is added, the last minute of the last day of the designated month has 61 seconds, which is displayed as "23:59:60." In the event of a negative leap second, the last second of a day would be "23:59:58."

ICU does not include leap seconds in its time calculations.