The History of Calendars
Gregorian Calendar
This calendar tool is based on the Gregorian calendar, the calendar used by most of the modern world.
The Gregorian calendar was decreed in 1582 CE by pope Gregory XIII, and immediately adopted in Italy, Spain, Portugal, France, the Netherlands, and in their territories and possessions.
The Gregorian calendar was gradually adopted in various other countries during the following 340 years, with several adoptions taking place as late as the 20th century, including Albania, Latvia, Lithuania, Bulgaria, Russia, Estonia, Romania, Yugoslavia, the USSR, Greece, Turkey, and China.
Julian Calendar
Prior to 1582 CE, most of the Christian world was on the Julian calendar, introduced by Roman Emperor Julius Caesar in 46 BCE as a replacement for the Roman calendar.
This tool shows accurate Gregorian dates. The Gregorian calendar is based on the Julian calendar, but the two calendars do not generally agree, and conversion between them is non-trivial.
About the Calendar Tool
When building calendars for years prior to about 1600 CE (Common Era, AD, or Anno Domini), check your dates to make sure they are Gregorian, and not based on some other calendar, such as the Julian.
Historical dates between 45 BCE and 1582 CE, are likely Julian dates, not Gregorian.
Dates shown on this Gregorian calendar tool generally will not match Julian dates.