Dates in the Julian and the Gregorian Calendars mostly have the same year number, e. For an explanation of the Julian day number displayed by the software see Julian Day Numbers. Software to convert between Julian and Gregorian Common Era calendar dates in various formats, and to perform date arithmetic.
Download the trial version. We number Julian days sequentially starting from Monday, January 1, B. The starting point is based on the harmony of at least three cycles. Astronomers developed the Julian day number system centuries ago to chronicle astronomical observations. Since observation usually occurred during the night, Julian Day Numbers JDN change during the day at noon for convenience.
Astronomers based the Julian Day Number system on a uniform time scale, but the earth's rotation varies very slightly and somewhat unpredictably. We measure the difference between the uniform Julian Day time and sundial time in seconds of difference.
We call this difference, Delta-T, and its value is fairly insignificant unless we are looking far back in time or ahead in the future. For example, in our study of the Elephantine Papyri , written during the fifth-century B. Our current Gregorian calendar is a slightly modified Julian calendar, which itself is a modified Egyptian calendar. The delay in switching meant that countries followed different calendar systems for a number of years, resulting in differing leap year rules.
In the Gregorian calendar, most years that are evently divisible by are common years, but they are leap years in the Julian calendar. This meant that the years , , and were leap years in countries still using the Julian calendar at the time e.
Two days were added to February , creating February 30, after the leap day in had erroneously been dropped, and the calendar was not synchronized with either the Julian or the Gregorian system.
By adding an extra leap day in , they were back on the Julian calendar. Both countries introduced the Gregorian calendar in In some non-western countries, the calendar reform took on many different guises to accommodate differing cultural and historical contexts. For example, Japan replaced its lunisolar calendar with the Gregorian calendar in January but decided to use the numbered months it had originally used rather than the European names.
However, the Nationalist Government formally decreed the adoption of the Gregorian calendar in China in January It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February ; the decree, a papal bull, is known by its opening words, Inter gravissimas. The reformed calendar was adopted later that year by a handful of countries, with other countries adopting it over the following centuries.
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