How was the seventh-day determined all around the world prior to the human devising and setting up of this so-called day/date line?
Were people traveling around the world prior to 1519 when Magellan's crew made the first such journey recorded in our history books? There is no record of it in the A.D. years. Prior to Columbus landing in America a lot of Europeans thought ships would fall off the edge of the world if they sailed too far from the European/Asian landmass.
The day started in the furtherest eastern landmass of Asia, traveled west over Asia and Europe.
Since the seventh-day Sabbath can be traced back to Christ who was in the tomb on the seventh-day Sabbath and rose on the first day, Sunday, it was carried both east and west by the apostles (as well as the Jews prior to the apostles).
As long as the gospel message was contained on the Asian/European/African landmass, there was no need to adjust for a dateline.
As long as European explorers sailed west when they went to the America's, or east around Africa when they sailed to Asia, they still didn't need or know of a dateline.
So yes, people simply determined what day it was by the rising and setting of the sun.
However, to "assume" that a 'dayline" is simply a "human invention", is just plain ignoring what obviously takes place on a round world. We've explained it a dozen times already. Yet we must be clear -- there is NO question as to what day it is in on any of the continents.
We have enough prophetic evidence to know what day is the Sabbath on all the continents.
We know that to sail from America to Australia results in a "six day week", and to sail back to America results in an "eight day week". Yet we also know that when traveling from Jerusalem east to Australia, the seven day week doesn't change. And when traveling from jerusalem west to America, the seven day week doesn't change.
That a day line exists was discovered when Ferdinand Magellan sailed around the world. And it was confirmed by other later sailors.
Prior to Magellan, explorers had "discovered" that another great ocean lay west of the America's. Europeans (who were after the spices and goods of Asia) started reasoning correctly that this great ocean reached Asia and if they sailed west to America and continued west across the second ocean then they could get to Asia.
Magellan decided to try it! He left Spain in 1519, reached America, sailed through the Straits of Magellan into the Pacific Ocean thinking it was just a short way to Asia. WRONG, it took three months to cross the Pacific. Magellan, himself never made it home, but a few of his crew did, being the first recorded to sail around the world.
But from that time on it became known that sailing across the Pacific cased the sailors to either lose or gain a day, depending on which way they travelled.
For the next 300 or so years there was no official dateline. Alaska, owned by Russia, was aligned with Russian time till America bought it.
The Philippeans, for a short time, were aligned with Mexico (America) time because of Spanish trade from Mexico, but moved back to eastern time in 1844.
The rest of the Southern Pacific Islands were all pretty well aligned with Australian time prior to 1884.
October, 1884
That is the date the international Meridian Conference took place in Washington D.C.
Greenwich England was chosen as the Prime Meridian (longitude 0) From Greenwich the longitudes are marked by degrees both east and west. A globe has 360 degrees, which makes 180th longitude exactly opposite to the prime Meridian. This became the nautical date line. The nautical date line, is not the same as the International Date Line. One is determined by mathematics, the other is determined by countries.
The later 1800's were an era of colonization with heavy american, English and German influence in the Pacific Islands, they one by one aligned themselves with the nautical date line. BUT not Tonga, Tonga had a strong enough government to resist the pressure and refused to change thus continuing to count their days according to eastern time (where all these Islands had been prior to 1884)