Coordinated universal time and the leap second

I will discuss the considerations that were important in the design of the current version of Coordinated Universal Time. The design includes the addition of additional “leap” seconds to keep Coordinated Universal time within +0.9 s of the UT1 time scale, which is a proxy for the position of the Earth in space. I will describe the advantages and problems associated with the leap-second system, and a number of changes that have been proposed to the realization of the time scale.