According to the position of the sun which, incidentally, was not new. Since ancient times the sun is used as reference for the calculation of time. The Egyptians already know: when it was overhead, was a sign that the day had arrived half. The technique is not the most accurate but relatively well resolved the issue.
Until the 19th century, every place acertava watches following own standards. The hours in Portugal were determined from the passage of the Sun by the National Observatory of Lisbon, the French used the Paris Observatory, and so on. This olhômetro generated bizarre situations: it was common watches nearby cities within the same country registering different times. This became a problem with the popularization of railroads - imagine crossing a large territory considering these random changes of time between the cities.
In 1884, 41 delegates from 25 countries attended a conference in Washington and only then decided that the meridian at Greenwich, England, would be the reference point for the calculation of longitudes. The rest of the history you learned in geography lessons: divided the Earth into 24 wedges of 15 degrees. Each of them represents a time difference from Greenwich. "Brazil just joined in fact the pattern in 1913," says Fadel David Anthony Son, geographer UNESP Rio Claro. Until then, Recife celebrating the new year 33 minutes before St. Paul and 1 hour and 5 minutes after the Porto Alegre.