To align our calculation of time with the irregular rotation of the earth, a ‘leap second’ is added every few years. On average the earth takes 2ms longer to rotate than the 86 400s we use as the length of a day. The time for a full rotation varies slightly and unpredictably from year to year. For this reason leap seconds do not follow a regular pattern and are only scheduled 6 months in advance. The last leap second was on 31 December 2016. There is no leap second currently scheduled. (http://bit.ly/3c7HEHN, http://bit.ly/38ZFbgr)