We all know that there is a three hour time difference between the West and East coast US. This means that if it is 9am in New York, it's just 6am in Los Angeles. It's not 6:01am in Los Angeles or 6:02am but exactly 6am. But how did this standardization come to be? The answer is it had a lot to do with the railroads...
Early in human history people would determine the time of day by the position of the sun in the sky. Moving into the middle ages sun dials were used to tell the time. The time would then frequently be displayed from the village clock. This is often known as true local time.
Because true local time is obviously the exact time at the location where the time was measured with a sun dial, it's easy to see how time differed from city to city, town to town, and even village to village. Towns just a few miles apart could differ in time by several minutes, with the discrepancy becoming larger the further apart two places were.
Now obviously this isn't good if you are trying to run a railway. Imagine trying to catch a train in one city at 1pm and arriving at your destination stop at 1pm. It just doesn't make sense! Because of this the railways came up with the idea of standardising and synchronising the time across entire regions.
The first country to standardize time was Britain, which uses GMT, or General Mean Time. The change happened on December 1st, 1847, but it then took until approximately 1855 the majority of town clocks in the UK were showing GMT. A funny side note here is that even after this time there was a famous church in Oxford which had two minute hands, one for GMT and one for local time.
Standard time was introduced to the US and Canada on November 18th, 1883. However, Detroit kept local time until 1900. Local time in Detroit is Standard Time minus 28 minutes.
So there you have it. A brief introduction to how the railroads brought standardization to time around the world. Time zone boundaries in the US have changed many times since they were introduced, and they continue to change today, although much less frequently.