Time Zones
Modern clocks are not set to the sun in the sky. Instead they are set against the current time of the current time zone. This abandoned the use of a local reference clock in favor of a regional clock. Effects of local time zones may need to be undone when setting up Bible Clocks for some uses.
Time Zone History
Before the invention of time zones each town or city usually kept its own reference clock. Time conversion tables were available that listed the difference in time between various important places.
To understand this situation better, consider the following chart. It was published in 1857. The purpose was to provide the raw data that was needed to convert times between any 2 cities listed in the table.
The reference city is Washington DC. Instructions at the bottom of the chart
explain how to convert times between Washington DC and any other city.
The same calculation is used 2 times to convert time between any 2 cities.
Each city was calculating a local time when the sun passed directly overhead. Setting time standards was done in each city. This is as it had been done for centuries. Nobody knew differently.
But note the problem that early railroad travelers had in this era. A train schedule would list times at stations in cities served. But the time between stations was not a simple difference between those times.
The whole ancient system of telling time by observation of the sun directly overhead did not work any more.
Great Western Railway
The change to standard time was begun by the Great Western Railway in Britain. Starting in 1840 they had adopted a single clock for their entire rail network. By 1847 they had tied their own clock to the clock used at the Greenwich Observatory. Telegraph wires were being used to set clocks across their network. By 1848 all railroads in Britain were using GMT as the basis for their train schedules so passengers had a chance of planning connections between companies.
It would take until 1890 before GMT was used across Britain for other purposes. Such was the resistance to using a standard clock and not the sun overhead.
US Railroad Standardization
Railroads in the USA held a conference in 1883 in order to agree on the first use of industry wide clocks for setting schedules. This untangled many of the problems of setting schedules for trains. It made trains safer to ride.
North America was more difficult to standardize because of the great size of the country compared to Britain. Especially traveling east and west the country would require more than 1 time zone. Every 15 degrees of westward travel retards the time of day the solar by 1 hour. To keep the time roughly the same would ultimately require 4 zones to cover the continental USA and Canada. The railroads were the first to adopt that convention.
International Meridian Conference
A year later the International Meridian Conference was held in October of 1884 in Washington DC. At this point the goal was to get the rest of the clocks across the country to agree with the clocks used by railroads.
It is important to understand the domain of this conference and what they agreed upon. Here is the list
It is desirable to have a single prime meridian for use by all nations. instead of the multiplicity then in use.
That the observatory of Greenwich should be that prime meridian for longitude.
That the unit of measure from that location should be counted out up to 180 degrees in both directions.
That there should be adopted a universal day. The purposes would be anything considered useful, but not to interfere with local days.
That universal day was to begin at the moment of mean midnight at the initial meridian. This would be the start of the civil day. Hours were to be counted from 0 to 24.
Astronomical and nautical days will be changed to start at midnight.
They finally expressed home in the use of decimal units to express the division of angular space.
The French would not adopt this system until 1911, and then would insist on a name change to "Coordinated Universal Time" instead of Greenwich Mean Time.
They also did not adopt any time zones, that would be left up to others. This conference adopted the reference time system that all other time zones around the world would eventually be measured against. Most, but not all, would be whole hours different from GMT.
Time shifts to midnight for the nautical day had been done in 1805 in Britain. By 1925 astronomical days were also shifted to midnight.
Note that shifting calendar day break to midnight meant that for most uses the calendar day shift would happen when most people are asleep. This had not been so in history. Scripture is written with a calendar day break at sunset.
By setting the calendar day break at midnight this convention set up the time keeping systems of the world for the international date line. This is always at the oppose side of the earth from the prime meridian.
Documentary
The following video gives a review of the situation in the USA before the adoption of time zones as we know them now.