Time

On a leap-second day, the last minute can have 61 seconds. To keep UTC close to Earth’s rotation, clocks may insert 23:59:60 after 23:59:59. Because that extra second can disrupt computer systems, global timekeepers plan to change the system by 2035.

AM stands for Ante Meridiem ("before noon" in Latin) and PM for Post Meridiem ("after noon"). Since 12:00 noon is noon itself—neither before nor after—the expressions "12 AM" and "12 PM" are inherently contradictory.