Twenty-fourths system - A method of calculating unearned premiums. Premiums accounted for in bulk on a monthly basis are assumed to attach on average on the middle day of each month. Therefore, at the end of a calendar year, 23/24ths of premium booked in January is assumed to be earned, with 24th unearned. 21/24ths of February policies earned, and 3/24ths unearned, and so on. See also Eighths system, and 365ths system. .