6 Strange Time Zone Disputes Time can be particularly complicated—especially when politics and war are involved

Um....

Okay: Accurate measure of time became important prior to the 19th Century, because if you have an accurate measure of time, you can accurately calculate your longitude. Because mechanical clocks weren't all that accurate then, especially if it was being moved around, people who needed accurate time would determine local time in relation to the sun as accurately as they could, then compare that with the transit of the moons of Jupiter. Seriously. They'd break out a table of transit times, see when it was supposed to transit relative to some point on the earth, then compare it to the local time to determine the longitude. With latitude and longitude, they could make accurate maps and know exactly where they were. Ships at sea would have to wait until mechanical clocks were accurate enough for the job and not effected by motion. Even on land, they could and did set mechanical clocks to a sundial and an almanac showing how far off the sun was on that day (some almanacs still list sun fast or slow relative to a specific location).

Something fun: Before settling on Greenwich, England, as longitude 0, each country picked their own longitude 0. I have a copy of an early 19th Century US map that lists longitudes relative to Washington, DC.

Out of curiosity, looked at Dublin, Ireland, and its longitude is -6.26028. That put it 25 minutes, 2 seconds earlier. Working back from 25 minutes, 21 seconds, that puts the place that was setting time for Ireland as -6.3375. That puts it just a little further west. That turns out to be the longitude of the Dunsink Observatory in Dublin, Ireland. Ireland's time was then the local time at Dublin, Ireland.