When I visited the Sinupang Pambansa or the Philippine National Archives in 2007 I saw and purchased a copy of the Catalogo Alfabetico de Apellidos, a reproduction of Governor-General Narciso Claveria's surname decree in 1849 with an introduction by Domingo Abella, then head of the PNA. In his write-up Abella enumerated the achievements of Claveria, whose short stint as chief executive of the Philippines proved to have several reforms that changed Philippine historiography for good.
What many people, especially those interested in Philippine Genealogy, don't know is that Claveria did more than just that famous surname decree of 1849. He had another decree, somewhat forgotten in many history books but whose effects still affect us today. In 1844 he made a decree that corrected the Philippine calendar. When the Spaniards first came to the Philippines they based their calendar on Spain's, not knowing that as they went westward and reached the Philippines this calendar fell a day behind the rest of world. So from the 1500s up until Claveria made a fuss about it in 1844 the people in the Philippines were actually living the wrong day every day! So Claveria, after consulting with the church, decreed that December 31, 1844 would not exist and instead December 30, 1844 would be immediately followed by January 1, 1845. So in one decree and erasure of one single day in the last month of the year the Philippines finally caught up with the rest of the world's calendar.
What does this do to genealogists researching Philippine families? Simple. Every record we've gone through before January 1, 1845 was one day behind. So if we'd put down, for example, Cotober 4, 1843 as the birthday of our ancestor then the ACTUAL birthday was October 5, 1843.
Perhaps it's simply a technical detail in Philippine genealogy, but a single day difference makes a lot of difference. Imagine how many people in our records have been given the wrong birthdays, marriages dates, and even death dates? Narciso Claveria, indeed, has proven to have the most impact in terms of Philippine genealogy.