Synchrony Despite Differences
All four calendars synchronize on the first day, 1 Tishrei of the year 1, 1 Tisrei 1, and in fact they synchronize until 29 Adar (I). After that, the Yerushalayim calendar and the new Hebrew calendar have a month of Adar Beth, whereas the Eretz Yisrael and the old Hebrew calendar have the month of Nissan. In jargon, year 1 is an LA2 year for the Yerushalayim and the new Hebrew calendar and a CA2 year for the Eretz Yisrael and the old Hebrew calendar.
This is the Molad of Tishrei of year 1, according to the four calendars. In the old Hebrew calendar, it is at day ב, at ה hours and 204 Chalakim. In both the calendars of Yerushalayim and Eretz Yisrael, the Molad of Tishrei 1 is at day ב, at ד hours and 642 Chalakim. The new Hebrew calendar has the first Molad at day ב, at י"ז hours, and 204 Chalakim.
And this is the Molad of Nissan of year 1, according to the four calendars. According to the old Hebrew calendar, it is at day ד, at ט hours and 642 Chalakim. According to the calendar of Eretz Yisrael, it is at day ד, at ט hours. As year 1 is a leap year in the Yerushalayim calendar, it is the Molad of Adar Beth that is at ד"ט. So, according to the Yerushalayim calendar, the Molad of Nissan 1 is at day ה, at כ"א hours, and 793 Chalakim, one month later. According to the new Hebrew calendar, it is at day ו, at י hours, and 355 Chalakim, which is 12 hours and 642 Chalakim later.
Exercise: explain the appearances of the three 642 Chalakim in the above. Hint.
To the memory of Harav Raziel Shevach, HY"D.
This is the Molad of Tishrei of year 1, according to the four calendars. In the old Hebrew calendar, it is at day ב, at ה hours and 204 Chalakim. In both the calendars of Yerushalayim and Eretz Yisrael, the Molad of Tishrei 1 is at day ב, at ד hours and 642 Chalakim. The new Hebrew calendar has the first Molad at day ב, at י"ז hours, and 204 Chalakim.
And this is the Molad of Nissan of year 1, according to the four calendars. According to the old Hebrew calendar, it is at day ד, at ט hours and 642 Chalakim. According to the calendar of Eretz Yisrael, it is at day ד, at ט hours. As year 1 is a leap year in the Yerushalayim calendar, it is the Molad of Adar Beth that is at ד"ט. So, according to the Yerushalayim calendar, the Molad of Nissan 1 is at day ה, at כ"א hours, and 793 Chalakim, one month later. According to the new Hebrew calendar, it is at day ו, at י hours, and 355 Chalakim, which is 12 hours and 642 Chalakim later.
Exercise: explain the appearances of the three 642 Chalakim in the above. Hint.
To the memory of Harav Raziel Shevach, HY"D.