Is It Coz I's Tick?

Here's an interesting story but I'm puzzled by this paragraph:

One Earth day is about 24 hours long. Over the course of a year, the length of a day normally changes gradually by one millisecond. It increases in the winter, when the Earth rotates more slowly, and decreases in the summer, Gross has said in the past.

I'm sure the answer's simple but I cannot see it. Perhaps some kind soul would enlighten me.