What exactly is a "day?" How do we know when a day has passed? According to astronomers, we mark the passage of days by the time between when the Sun is highest in the sky.
Oddly enough, however, that time period isn't constant. Days are a bit longer or shorter than 24 hours. This page calculates the actual length of the days for any year.
Who knows, perhaps you can use this information to bill the boss for a few more cents! (Or, if she sees this page, to dock you a few cents...)