How Do We Calculate Our Calendar?

The Beginning of The Year: Ex. 12:1-2; Ex. 13:4; Ex. 23:15 – According to the Aviv (Barley) and not the Spring Equinox; not Tishrei/The 7th Month (according to the Babylonian Calendar).
The Beginning of The Month: New Month/Moon (Rosh Chodesh): Psalm 81:1-4, Isaiah 66:23
The Moedim (Appointed Feasts): Leviticus 23:1-4.
All Moedim listed in the Bible, begin the night before, at sundown. We have compensated for this by including both dates on the Schedule. Therefore, making the observance, from the sundown listed, until the next sundown listed.
Our dates may or may not differ from the Orthodox Jewish Calendar.