Every four years we have a leap day, February 29th, because there are about 365.25 days in each year.
And sometimes we need to have a leap second. There should be 24 hours in each day, which is the time it takes for the earth to spin one time. But the Earth is natural, and sometimes it spins just a little faster or slower. So today we will have a leap second and there will be one more second in the day, and New Year’s Day will start one second late.
This causes many problems for clocks and computers. Next year is the 150th anniversary of one of the most famous clocks in the world, the Great Clock Tower in London (with the famous Big Ben bell).
The original pendulum is still used to keep the time, and the clockmaster uses old pennies on the pendulum in order to slow it down or speed it up. He will have a tough time tonight.