Julian calendar

The Julian calendar, proposed by Julius Caesar in 46 BCE (708 AUC), was a reform of the Roman calendar. It was first used in 1 January 45 BCE. It was the main calendar in most of the world, until Pope Gregory XIII replaced that with the Gregorian calendar in 4 October 1582.

2024 in other calendars

Gregorian calendar	2024 MMXXIV
Ab urbe condita	2777
Armenian calendar	1473 ԹՎ ՌՆՀԳ
Assyrian calendar	6774
Bahá'í calendar	180–181
Balinese saka calendar	1945–1946
Bengali calendar	1431
Berber calendar	2974
British Regnal year	2 Cha. 3 – 3 Cha. 3
Buddhist calendar	2568
Burmese calendar	1386
Byzantine calendar	7532–7533
Chinese calendar	癸卯年 (Water Rabbit) 4720 or 4660     — to — 甲辰年 (Wood Dragon) 4721 or 4661
Coptic calendar	1740–1741
Discordian calendar	3190
Ethiopian calendar	2016–2017
Hebrew calendar	5784–5785
Hindu calendars
- Vikram Samvat	2080–2081
- Shaka Samvat	1945–1946
- Kali Yuga	5124–5125
Holocene calendar	12024
Igbo calendar	1024–1025
Iranian calendar	1402–1403
Islamic calendar	1445–1446
Japanese calendar	Reiwa 6 (令和６年)
Javanese calendar	1957–1958
Juche calendar	113
Julian calendar	Gregorian minus 13 days
Korean calendar	4357
Minguo calendar	ROC 113 民國113年
Nanakshahi calendar	556
Thai solar calendar	2567
Tibetan calendar	阴水兔年 (female Water-Rabbit) 2150 or 1769 or 997     — to — 阳木龙年 (male Wood-Dragon) 2151 or 1770 or 998
Unix time	1704067200 – 1735689599

During the 20th and 21st centuries, the date according to the Julian calendar is 13 days behind the Gregorian date.

Year length; leap years

The Julian calendar has two types of year: common years of 365 days and leap years of 366 days. There is a simple cycle of three common years followed by a leap year and this pattern repeats forever. However, the rule was not followed in the first years after the reform in 45 BCE. Due to a counting error, every 3rd year was a leap year instead of the 4th. The leap years were:

• 45 BCE (709 AUC)
• 42 BCE (712 AUC)
• 39 BCE (715 AUC)
• 36 BCE (718 AUC)
• 33 BCE (721 AUC)
• 30 BCE (724 AUC)
• 27 BCE (727 AUC)
• 24 BCE (730 AUC)
• 21 BCE (733 AUC)
• 18 BCE (736 AUC)
• 15 BCE (739 AUC)
• 12 BCE (742 AUC)
• 9 BCE (745 AUC)

However, in 8 BCE (746 AUC), emperor Augustus Caesar corrected the problem. The next leap year was 7 CE (160 AUC).

Criticism

With the simple cycle, the length of the Julian year is exactly 365.25 days (365 days and 6 hours), but the actual time it takes for the Earth to go around the Sun once is closer to 365.2422 days (about 365 days, 5 hours, 48 minutes and 46 seconds). This difference is about 365.25 - 365.2422 = 0.0078 days (11 minutes and 14 seconds) each year, although Greek astronomers knew that. This made the seasons get out of track, since the real first day of spring in western Europe (the equinox - day and night the same length) was happening earlier and earlier before the traditional 21 March as the centuries went by. By the 1500s, it was starting around 11 March, ten days 'too early' according to the calendar.

Reform

From Roman calendar

The first step of the reform was to realign the 25 December with the Winter solstice by making 46 BCE (708 AUC) 445 days long. In ordinary Roman calendar, the common year had 355 days and the leap year (one year after the common year) had 378 days. The 46 BCE was a leap year, according to the calendar. Julius Caesar added 67 more days by adding two extra months (those are called Prior and Posterior in letters of Cicero) between November and December.

Sources: [1], [2]

Months	47 BCE (707 AUC)	46 BCE (708 AUC)	45 BCE (709 AUC)	8 BCE (746 AUC)
January	29	29	31	31
February	28	24	30	28
Intercalaris		27
March	31	31	31	31
April	29	29	30	30
May	31	31	31	31
June	29	29	30	30
Quintilis	31	31	31	31
Sextilis	29	29	30	31
September	29	29	30	30
October	31	31	31	31
November	29	29	30	30
Prior		33
Posterior		34
December	29	29	31	31
Total	355	445	366	365