Whence Henry III provide for how to count the increasing day (February 29)

King Henry III in 1236 referred to February 29 as the “increasing day”, and the statute at Westminster from 1236 is remarkably the same as the current North Carolina law, which now provides (GS 12-3) “In every leap year the increasing day and the day before, in all legal proceedings, shall be counted as one day.” The law cited in its history note is 21 HENRY III — “STATUTE DE ANNO ET DIE, BISSEXTILI” and stated  “the day increasing in the Leap-Year shall be … taken and reckoned of the same month wherein it groweth; and that day, and the day next going before shall be accounted for one day …” (though commentators disagree whether this was an act of Parliament or a writ of King Henry III, nonetheless it is carried in the Statutes at large for that Parliament)