Time does change. Our common (civil) time is defined as a full rotation of our planet relative to our sun, but Earth wobbles a bit so every day is a little bit longer or shorter, and we are generally slowing down, ever so slowly.

The International Earth Rotation and Reference Systems Service publishes the Earth’s daily rotation speeds, and they decide when we should have leap seconds snuck into the official clocks of the world to account for the wobbliness.

If you want to learn more about leap seconds, there is a great article about them here: http://queue.acm.org/detail.cfm?id=1967009