Keeping Track of the Helliacal Risings

By 600 BC the Babylonians had discovered a regular pattern that seemed to keep the months more or less in sync with the seasons. This cycle, called the metonic cycle, lasted 19 years and in it the third, sixth, eighth, eleventh, fourteenth, seventeenth and nineteenth years all had the extra thirteenth month. So over the course of 19 years you would have 235 months. The calendar drifts by only one day every two hundred and nineteen years.