A leap second is an additional second added to Coordinated Universal Time (UTC) to keep it in sync with the Earth's rotation. The Earth's rotation is not perfectly constant, so over time, it can drift from atomic time, which is based on precise measurements of time using atomic clocks. Leap seconds are typically added either on June 30 or December 31. The decision to add a leap second is made by the International Earth Rotation and Reference Systems Service (IERS) based on observations of the Earth's rotation. This adjustment ensures that our clocks remain aligned with the position of the Earth relative to the Sun.