A leap second is an additional second added to Coordinated Universal Time (UTC) to correct for the discrepancy between the cesium atomic clock standard and solar time based on the Earth's actual rotation and revolution speed. Currently, leap seconds are added to maintain the difference between UTC and solar time within ±0.9 seconds. It is added either at the end of December 31st or at the end of June 30th.
TAI and POSIX time ignore leap seconds and always assume a day is 86,400 seconds. UTC can have 86,399 seconds due to leap seconds, and since TAI does not adjust its time when leap seconds occur, the difference between TAI time and UTC time accumulates over time. UTC considers leap seconds because it's based on astronomical standards. While both POSIX and TAI ignore leap seconds, TAI is a linear time that accurately measures real seconds, while POSIX is a UTC-based time that ignores leap seconds. Since 1972, starting with 10 seconds, 27 additional seconds have accumulated, making UTC has been exactly 37 seconds behind TAI.
Smearing is a method of gradually adjusting time to prevent large jumps when leap seconds occur. This approach is used to reduce discrepancies caused by leap seconds without directly reflecting them in POSIX time. However, the time adjustment method is not standardized and can be implemented differently by different systems (typically by distributing one second over a day). Due to this complexity and a major system failure in 2012, there are movements to eliminate leap seconds altogether until 2035.
