The dates shown have to be confirmed by the sighting of the New Moon.
https://kingdomtruther.com/having-assurance-of-the-biblical-calendar/
https://kingdomtruther.com/the-biblical-calendar
https://kingdomtruther.com/the-13th-month-of-the-biblical-calendar/
First New Moon after the vernal Equinox is day 1 of the New Year