Before time became standardized, clock masters used “True Time”. The day work began and ended with the sun. This time period was divided into 12 equal hours. This meant that these hours would vary with the seasons, as the length of daylight changed. Each town would have their own variance of this “True Time”. Eventually, cities adopted “Mean Time”, which is how we think of time nowadays. Astronomers used the Earth's rotation and the stars to calculate the time, and divided the day into 24 uniform and equal hours. Geneva was the first city to adopt mean time in 1780, followed by London in 1792, Berlin in 1810, Paris in 1816, and Vienna in 1823.