The Hebrew calendar keeps its lunar months in step with the solar year by adding a whole extra month seven times in every nineteen years.

Nineteen solar years and 235 lunar months agree to within a couple of hours, which is the Metonic cycle, and three separate civilisations found it independently. Before the calculation was fixed in writing, the decision to add the month was taken by a court that considered whether the barley was ripe and whether the roads were passable for pilgrims.