Hebrew Calendars are ubiquitous in todays world, published and digital. Yet few understand the process behind them. 

At the epicenter of the calendar is the Molad, we anounce it on Shabbat Mevarchim, yet its relationship with the calendar is obscure, perhaps for good reason. The Molad which is the determinent of the Hebrew Calendar is the Molad of Tishrei, the one we don’t announce.

When I refer to the Hebrew Calendar I mean the Fixed Hebrew Calendar. Why “fixed”, because Molads are separated by 29 days 12 hours and 793 parts (out of 1080 parts in an hour). Some call this the מולד אמצעי. 

Without the Molad there is no Fixed Hebrew Calendar. The Fixed Hebrew Calendar is based on the Molad and not on the sighting of the moon. The sighting of the moon has not been a factor in the determination of the Hebrew date since the inception of the Fixed Hebrew Calendar.

What is fixed in the Fixed Hebrew Calendar is the interval between consecutive months. 

What makes us associate the Molad with the moon is