Long before the Greenwich meridian became the de facto prime meridian and, by consequence, placed the calendrical date line in the waters of the Great South Sea (as the Pacific Ocean was then commonly known), its American and Asian perimeters already had a long history of keeping different day counts.

The origin for this calendrical anomaly was the demarcation line proposed in 1493 by Pope Alexander VI in the bull Inter Caetera. This line, defined as the meridian situated 100 leagues west of the Cape Verde Islands, would divide the spheres of influence of the Spanish crown (westwards of the demarcation line) and the Portuguese crown (eastwards of the same line). In the following year, the demarcation line was shifted 370 leagues west of the Cape Verde Islands in the Treaty of Tordesillas, a modification that was sanctioned by Pope Julius II in 1506.

The Colonization of the Pacific Ocean

European explorers who approached the Pacific Ocean by sailing to the east such as the Portuguese, and in their wake the Dutch, the English and the French, naturally kept their ships journals and diaries according to the day count of their home land and this was of course also adopted by the colonists who settled along the Asian perimeter of the Pacific Ocean.

However, the colonization of the Pacific Ocean by the Spanish occurred from the opposite direction and more specifically from the Spanish possessions in America. The Philippine archipelago was discovered in March 1521 by Ferdinand Magellan and Spanish dominion over the islands was first firmly established in 1565 by Miguel López de Legazpi (1502-1572), the conquistador and first Spanish governor-general of the Philippines. He had been equipped with five ships by Luis de Velasco, the viceroy of New Spain, and left Acapulco in 1564. In April 1565 he reached Cebu, one of the southern islands of the archipelago, and founded the first Spanish settlement on the site of modern Cebu City. In 1570 he sent an expedition to the northern island of Luzon and in the next year he founded the city of Manila, which became the capital of the new Spanish colony and Spains major trading port in East Asia.

Most of the shipping from the Philippines to Spain went over the Pacific Ocean to the Mexican port of Acapulco, was transported overland to the port of Veracruz, and then shipped across the Atlantic to Spain. In order that the Spanish ships crossing the Pacific Ocean between the Philippines and the Spanish Americas would not have to adjust the dates in their journals whenever they sighted land, the Philippines observed the same day count as that of the Spanish Americas.

The Philippine tradition of counting the days according to the American reckoning was explained in great detail in 1590 by the Spanish historian José de Acosta (1539-1600) in his Historia natural y moral de las Indias:

[...] seeing the two Crownes of Portugall and Castille, have met by the East and West, ioyning their discoveries together, which in truth is a matter to be observed, that the one is come to China and Iappan by the East, and the other to the Philippines, which are neighbours, and almost ioyning vnto China, by the West; for from the Ilands of Lusson, which is the chiefe of the Philippines, in the which is the Citie of Manilla, vnto Macao, which is in the Ile of Canton, are but foure score or a hundred leagues, and yet we finde it strange, that notwithstanding this small distance from the one to the other, yet according to their accoumpt, there is a daies difference betwixt them; so as it is Sunday at Macao, whenas it is but Saturday at Manilla, and so of the rest. Those of Macao and of China have one day advanced before the Philippines.

It happened to father Alonso Sanches [...] that parting from the Philippines, he arrived at Macao the second day of Maie, according to their computation, and going to say the Masse of S. Athanasius, he found they did celebrate the feast of the invention of the holy Crosse, for that they did then reckon the third of Maie. The like happened vnto him in another voyage beyond it.

Some have found this alteration and diversitie strange, supposing that the fault proceedes from the one or the other, the which is not so; but it is a true and well observed computation, for according to the difference of waies where they have beene, we must necessarily say, that when they meete, there must bee difference of a day; the reason is, for that sailing from West to East, they alwaies gaine of the day, finding the sunne rising sooner; and contrariwise, those that saile from East to West, do alwaies loose of the day, for that the Sunne riseth later vnto them; and as they approach neerer the East or the West, the have the day longer or shorter. In Peru, which is westward in respect of Spaine, they are above sixe houres behinde; and when it is morning heere, it is mid-night there. I have made certaine proofe thereof, by the computation of Eclipses of the Sunne and Moone. Now that the Portugalls have made their navigations from West to East, and the Castillians from East to West, when they came to ioyne and meete at the Philippines and Macao, the one have gained twelve houres, and the other hath lost as much; so as at one instant, and in one time, they finde the difference of twentie[-four] houres, which is a whole day; so as necessarily, the one are at the third of Maie, whenas the others accoumpt but the second; and whenas the one doth fast for Easter eve, the others eate flesh for the day of the resurrection.

And if we will imagine that they passe farther, turning once againe about the world, vsing the same computation, when they should returne to ioyne together, they should finde by the same accoumpt, two daies difference; for as I have saide, those that go to the Sunne rising, accoumpt the day sooner, for that the Sunne riseth to them sooner, and those that go to the setting, accoumpt the day later, for that it goes from them later; finally, the diversitie of the noone tide causeth the divers reckoning of the day. And now for as much as those that doe saile from East to West, change their noone tide without perceiving it, and yet still follow the same computation they did when they parted, of necessitie, having made the compasse of the worlde, they must finde the want of a whole day in their computation.

William Dampier

William Dampiers Description of the Philippine Day Reckoning

When the British ex-buccaneer, sea-captain and explorer William Dampier (1651-1715), during his travels across the globe from 1679 to 1691, weighed anchor after his stop-over on the island of Mindanao in the Philippines in January 1687, he noted in his journal:

It was during our stay at Mindanao, that we were first made sensible of the change of time, in the course of our Voyage. For having travelld so far Westward, keeping the same Course with the Sun, we must consequently have gaind something insensibly in the length of the particular Days, but have lost in the tale, the bulk, or number of the Days or Hours. According to the different Longitudes of England and Mindanao, this Isle being West from the Lizzard, by common Computation, about 210 Degrees, the difference of time at our Arrival at Mindanao ought to be about 14 Hours: And so much we should have anticipated our reckoning, having gained it by bearing the Sun company. Now the natural Day in each particular place must be consonant to itself: But this going about with, or against the Suns course, will of necessity make a difference in the Calculation of the civil Day between any two places.

Accordingly, at Mindanao, and all other places in the East-Indies, we found them reckoning a Day before us, both Natives and Europeans; for the Europeans coming eastward by the Cape of Good Hope, in a Course contrary to the Sun and us, where-ever we met they were a full Day before us in their Accounts. So among the Indian Mahometans here, their Friday, the Day of their Sultans going to their Mosques, was Thursday with us; though it were Friday also with those who came eastward from Europe. Yet at the Ladrone Islands, we found the Spaniards of Guam keeping the same Computation with our selves; the reason of which I take to be, that they settled that Colony by a Course westward from Spain: the Spaniards going first to America, and thence to the Ladrones and Philippines.

The Philippine Adjustment of 1844/45

During the early 1840s the commercial interests of the Philippine Islands turned more and more away from the Spanish Americas (which for a large part had severed their relations with the mother land Spain) and trading with the Chinese mainland (engendered by the ignominious but lucrative Opium Wars), the Malay peninsula, the Dutch East Indies and Australia became increasingly important.

In order to facilitate communication and trading with its western and southern neighbours, the secular and religious authorities of the Philippines agreed that it would be advantageous to abolish the American day reckoning and adopt the Asian day reckoning. This was achieved in 1844 when Narciso Claveria, the governor-general of the Philippines, issued a proclamation announcing that Monday, 30 December 1844, was to be immediately followed by Wednesday, 1 January 1845.

Outside of the Philippines, the fact of the adjustment was little noticed and up to the early 1890s many European atlases and geographical handbooks still maintained that the Philippines observed the American day reckoning.