A Rainy Request & Avra(ha)m’s Journey

Why is there a different starting date for T&M in the Diaspora? Why is the date in Israel on the Jewish calendar and in Chutz LaAretz on the secular one?

Why is there a different starting date for T&M in the Diaspora? Why is the date in Israel on the Jewish calendar and in Chutz LaAretz on the secular one?