Why did Babylonian calendars need extra months?

The Babylonian calendar was based on a lunar year of 12 synodic months, totaling approximately 354 days. Because this lunar year is shorter than the solar (or agricultural) year of approximately 365 days, the calendar needed to be reconciled with the seasons to ensure that agricultural activities and festivals remained aligned with the correct time of year.1234

To bridge this discrepancy, the Babylonians used intercalation, the periodic addition of an extra month. Initially, these intercalations were performed haphazardly based on royal decree or perceived need. However, by approximately 380 bce, Babylonian astronomers established a more systematic approach using a 19-year cycle—similar to the Metonic cycle—in which an extra month was added in seven out of every 19 years. This ensured that the first month of the year, Nisanu, remained near the onset of spring.134