The Millerites were the followers of the teachings of William Miller, who in 1831 first shared publicly his belief that the Second Advent of Jesus Christ would occur in roughly between 1843 and 1844.
The beginning of the 2,300 days: The decree of Artaxerses in the 7th year of his reign (457 BC) as recorded in Ezra marks the beginning of the 2300 days. King's reigns were counted from New Year to New Year following an Accession Year. The Persian New Year began in Nisan (March–April). The Jewish civil New Year began in Tishri (September–October).