Well it's only birthday law in the sense that it has to do with my birthday, but, hey, it's my blog. The law (well, former law): Leap year day and the day before it are one day. (True, at the time leap year day wasn't necessarily my own February 29, but the law remained in effect into the era of the February 29 convention.) Here's the scoop, from the Australian Lex Scripta site:
By a Statute of Henry III, entitled de Anno et die Bissextili, it was provided that, in a leap year, the bissextile day "and the Day next going before, shall be accounted for one Day". The rational compulsion behind this legislative fiction has been lost in the mists of time. Indeed, it is not even certain when it was enacted: some references assign to it the regnal year 21 Henry III (that is, 1236), whilst others assign to it the regnal year 40 Henry III (that is, 1256).
In the time of Lord Coke, at least, the fiction of treating the bissextile and the preceding day as a single day was alive and well. In England, the Act of Henry III was not repealed until 1879. In some Australian States, it has also been expressly repealed: for example, in New South Wales in 1969. Where it has not been expressly repealed, the suggestion is that it has no continuing application, perhaps because it was impliedly repealed by the adoption of the Gregorian Calendar in 1751 [the year in which the Act for Regulating the Commencement of the Year and for Correcting the Calendar now in Use was enacted; the Act took effect in 1752 -EV].
The point arose before Burchett J. in Re Clubb; ex parte Clubb v. Westpac Banking Corporation, in which the respondent bank sought to rely on the statute of Henry III to justify its method of charging interest on customers' accounts. The evidence showed that the bank had divided its annual interest rate by 365, and added this amount of interest to the customer's account for each day, producing the result that, in a leap year, the customer was charged an extra day's interest over the agreed annual rate. Not surprisingly, Burchett J. held that this was impermissible ....