leap second

intentional time step of one second [...] to adjust UTC [...] to ensure appropriate agreement with UT1, a time scale [...] based on the rotation of the Earth Note 1 to entry: See also ITU-R TF.460-6. Note 2 to entry: An inserted second is called a positive leap second and an omitted second is called a negative leap second. A positive leap second is inserted after [23:59:59Z] and can be represented as [23:59:60Z]. A negative leap second is achieved by the omission of [23:59:59Z]. Insertion or omission takes place as determined by the International Earth Rotation and Reference Systems Service (IERS), normally on 30 June or 31 December, but if necessary on 31 March or 30 September.