Originally Posted by
monogroover
Before the advent of the railways and their timetables, there was no single unified timezone in Britain. If it was 6pm in London, it probably wasn't 6pm in Newcastle. What the definitive reference for the time was in any town or city back then, I have no idea. The cathedral or town hall clock? How did whoever was responsible for maintaining those in the early 19th century determine the correct time?
Link:
Before clocks were invented, people kept time using different instruments to observe the Sun’s meridian passing at noon. The earliest time measuring devices we know of are sundials and water clocks...
...and having observed the meridian passage they would be able to correct the town / church clock. Once they had some idea of the clocks timekeeping ability they would be able to apply an average correction until they made another accurate solar observation.