This posting was made on January 3, 2021, according to my computer. And, if you ask someone what day it is and you are likely to get this same date (if you read the post on that day). This date is the one that based upon the Gregorian Calendar, the one commonly used by the United States and most of the world. The Gregorian Calendar is the currently accepted International Calendar date. But, did you know that there are at least four other calendars commonly in use? They are the Julian, Hebrew, Islamic, and Chinese. So, January 3, 2021 could really be December 21, 2020. (Julian Calendar with the Julian day being 2459217.5), or Teveth 19, 5781 (Hebrew), Jumada I-Ula 19, 1442 (Islamic) or Ding-Chou(Ox) (11th month), 20, 4718 (Chinese). So, it can be a bit confusing. These calendars are still in use for a variety of reasons.
The Georgian Calendar, while the International Standard today, was not widely accepted until after 1752 when Britain and the British Colonies accepted the correction to the Julian Calendar. This calendar is named for Pope Gregory XIII, who proclaimed its use in 1582. Astronomers still use the Julian Calendar. The Julian Calendar is based on the one adopted by Julius Caesar in approximately 46 BC with a few corrections and finalized in 8 CE. The Georgian Calendar is a slight correction to the Julian Calendar in how it manages leap years. The average length of the Georgian year is 365.2425 days as measured from equinox to equinox, i.e. a solar tropical year. Using this calendar the error is only 1 day in every 3300 years as compared to the Julian Calendar which accumulates an error of 1 day in every 128 years. (The advantage of the Julian Calendar to astronomers is that all of the known astronomical observations have positive Julian dates, making the math easier.)
Confused? Maybe, but then most of us don’t have to switch between calendar systems. So, knowing the time of day is much easier right? Not really, there either. Time is a fundamental measurement; a second is defined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.” Sure, everyone knows exactly what that means, right? Think of it this way, an electron “hops” from one level to the next and back again, 9,192,631,770 times in 1 second. It behaves like a very fast pendulum. This definition of a second wasn’t accepted until a meeting in 1967. Up until then the definition of a second was based upon the length of one solar day; however, there was a debate on the exact length of a solar day.
Time and the synchronization of time is actually relatively new. For most communities, time was a local issue. As commerce expanded, time and location became linked in what was known as the longitude problem, i.e. where was a ship located east or west of a given line. The problem was solved by using sophisticated mechanical clocks measured against a reference. Hence, the establishment of Greenwich Mean Time or the GMT. The longitude problem was officially solved, as recognized by the awarding of a prize by the British Government, in 1773. Yet, standardization of time was still at least a century away. What finally made it an imperative to establish a standard time and time zones? The advent of the railroads. At noon on November 18, 1883, the U.S. and Canadian Railroads adopted what was then called the General Time Convention, which later became the known as “Railroad and Telegraph Time.” The General Time Convention used the meridians (longitude) to establish time zones. The Naval Observatory was responsible for providing the noon signal which allowed the railroads and telegraphs to synchronize the clocks based upon the convention established. This occurred prior to formal establishment of GMT being the international standard at zero longitude. The international agreement came in 1884. Today, GMT is also referred to as Universal time (UTC) or Zulu time.