An occasional adjustment of one second, added to, or subtracted from, Coordinated Universal Time (UTC) to bring it into approximate synchronism with UT1, which is the time scale based on the rotation of the Earth. Note 1: Adjustments, when required, are made with respect to the last second of the last minute of the day (Universal Time) on one of two annual days of opportunity (June 30 or December 31. ) Note 2: The last minute of the day on which an adjustment is made will therefore have 61 or 59 seconds. The former is the usual case, and the latter, a theoretical possibility if ever needed. Note 3: An adjustment is required on any day of opportunity when it is anticipated that if unadjusted, UTC will deviate from UT1 by more than 0. 9 second before the next day of opportunity.