1. Calendar

2. Julian calendar • The Julian calendar, introduced by Julius Caesar in 46 BC (708 AUC). • This was a reform of the Roman calendar. It took effect in 45 BC (709 AUC). • The Julian calendar has a regular year of 365.25days divided into 12 months • Consequently, the Julian calendar is currently 13 days behind the Gregorian calendar; for instance, 1 January in the Julian calendar is 14 January in the Gregorian. • However, most branches of the Eastern Orthodox Church still use the Julian calendar for calculating the dates of moveable feasts, including Easter (Pascha). • The Julian calendar is still used by the Berber people of North Africa and on Mount Athos. In the form of the Alexandrian calendar, it is the basis for the Ethiopian calendar, which is the civil calendar of Ethiopia.

3. Gregoriancalendar • The Gregorian calendar, also called the Western calendar and the Christian calendar • The calendar was a refinement in 1582 to the Julian calendar amounting to a 0.002% correction in the length of the year. • It has been the unofficial global standard for decades, recognized by international institutions such as the United Nations and the Universal Postal Union

4. The Gregorian calendar is a solar calendar. A regular Gregorian year consists of 365 days and in a leap year, an intercalary or leap day is added as 29 February making the year 366 days. • Gregorian years are identified by consecutive year numbers. The cycles repeat completely every 146,097 days, which equals 400 years, and which also happens to be 20,871 seven-day weeks. • Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A calendar mean year is 365+97/400 days = 365.2425 days = 365 days, 5 hours, 49 minutes and 12 seconds. The same result is obtained by summing the fractional parts implied by the rule: 365 + 1⁄4 − 1⁄100 + 1⁄400 = 365 + 0.25 − 0.01 + 0.0025 = 365.2425 • Normally a leap year occurs every 4 years, but the Gregorian calendar omits 3 leap days every 400 years, unlike the Julian calendar, which retains those leap days. • Example : 1900 is not a leap year but 2000 is leap year.