Difference Between Gregorian and Julian Calendar Dates
Since the introduction of the Gregorian calendar, the difference between Gregorian and Julian calendar dates has increased by three days every four centuries:
| Gregorian range | Julian range | Difference |
|---|---|---|
| From 15 October 1582 to 10 March 1700 |
From 5 October 1582 to 28 February 1700 |
10 days |
| From 11 March 1700 to 11 March 1800 |
From 29 February 1700 to 28 February 1800 |
11 days |
| From 12 March 1800 to 12 March 1900 |
From 29 February 1800 to 28 February 1900 |
12 days |
| From 13 March 1900 to 13 March 2100 |
From 29 February 1900 to 28 February 2100 |
13 days |
| From 14 March 2100 to 14 March 2200 |
From 29 February 2100 to 28 February 2200 |
14 days |
A more extensive list is available at Conversion between Julian and Gregorian calendars.
This section always places the intercalary day on 29 February even though it was always obtained by doubling 24 February (the bissextum (twice sixth) or bissextile day) until the late Middle Ages. The Gregorian calendar is proleptic before 1582 (assumed to exist before 1582) while the Julian calendar is proleptic before year AD 1 (because non-quadrennial leap days were used between 45 BC and AD 1).
The following equation gives the number of days (actually, dates) that the Gregorian calendar is ahead of the Julian calendar, called the secular difference between the two calendars. A negative difference means the Julian calendar is ahead of the Gregorian calendar.
where is the secular difference; is the hundreds digits of the year using astronomical year numbering, that is, use (year BC) − 1 for BC years; and is integer division of H by 4. This truncates (removes) any decimal fraction (remainder) of the division, for both positive and negative numbers. Thus during the 1900s, 19/4 = 4, while during the −500s, −5/4 = −1.
The calculated difference increases by one in a centurial year (a year ending in '00) at either 29 February Julian or 1 March Gregorian, whichever is later. For positive differences, 29 February Julian is later, whereas for negative differences, 1 March Gregorian is later.
Read more about this topic: Gregorian Calendar
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