[TedDoesntTalk]

Wikipedia says the international date line is “a cartographic convention, and is not defined by international law.”

The “line” wasn’t a convention at all in the 16th century so how did sailors experience loss of a day?

https://en.m.wikipedia.org/wiki/International_Date_Line#:~:t....

[canjobear]

The line is the reason you don’t lose a day now. When you circumnavigate the world westward, the earth has rotated one less time for you than for everyone else, so it appears you have lost a day. By artificially switching the day at the date line you avoid this kind of slippage.

[psunavy03]

On a ship, you also keep the same time zone as local time for the area you are steaming through. Going consistently westward to get somewhere is a bonus, because every other day you get to sleep in an extra hour.

[rcxdude]

Because the count of the days for the people they encountered was derived by traveling in the other direction around the earth.