At the end of the day on June 30 this year there will be a leap second. That is, we will insert an additional second into our time reckoning. This will happen after 23:59:59 Coordinated Universal Time (UTC), which corresponds to 7:59:59 PM, Eastern Daylight Time. If you don’t update your clocks at that time, your clocks will be one second fast. OK, this won't matter to most people, but there is a fascinating connection to creation in this story. Throughout history, the basis for time measurement primarily has been the motion of the earth. For instance, the earth's daily rotation defines the day. We subdivide the day into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. Hence, there are 86,400 seconds in a day. However, the earth's rotation is not a perfect time keeper, for its rotation changes ever so slightly. It was not until recent times that technology has allowed us to detect the minute changes in the earth's rotation. For most people, this is of no consequence. But for some experiments that require extremely accurate time measurements, the earth’s rotation is not a good standard to measure time.
In the 1950s, scientists defined the second to be 1/31,556,925.9747 of the mean tropical year in the year 1900. This standard was adopted about the time that atomic clocks were developed, and scientists quickly realized that atomic clocks offered the possibility of a more dependable standard of time. In 1967, scientists redefined the second to be 9,192,631,770 cycles of a particular transition of the caesium-133 atom. This definition closely matched the length of the second then in use.