The Longest Year in History (2008)

Using the best astronomy and mathematics of the time, Caesar tinkered the old Roman calendar to make it fit the reality of the solar year. The priests of Rome juggled a shifting 12-month calendar that ran at least ten days behind the solar year. Also at the far right a month is labeled INTER, which stands for the extra intercalary month added every two years or so to make the calendar stay in step with the solar year.

Source: www.uh.edu