Deaths
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| Millennium: | 1st millennium BC |
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| Centuries: | |
| Decades: | |
| Years: |
| 21 BC by topic |
| Politics |
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| Categories |
| Gregorian calendar | 21 BC XX BC |
| Ab urbe condita | 733 |
| Ancient Greek era | 189th Olympiad, year 4 |
| Assyrian calendar | 4730 |
| Balinese saka calendar | N/A |
| Bengali calendar | −613 |
| Berber calendar | 930 |
| Buddhist calendar | 524 |
| Burmese calendar | −658 |
| Byzantine calendar | 5488–5489 |
| Chinese calendar | 己亥年 (Earth Pig) 2676 or 2616 — to — 庚子年 (Metal Rat) 2677 or 2617 |
| Coptic calendar | −304 – −303 |
| Discordian calendar | 1146 |
| Ethiopian calendar | −28 – −27 |
| Hebrew calendar | 3740–3741 |
| Hindu calendars | |
| - Vikram Samvat | 36–37 |
| - Shaka Samvat | N/A |
| - Kali Yuga | 3080–3081 |
| Holocene calendar | 9980 |
| Iranian calendar | 642 BP – 641 BP |
| Islamic calendar | 662 BH – 661 BH |
| Javanese calendar | N/A |
| Julian calendar | 21 BC XX BC |
| Korean calendar | 2313 |
| Minguo calendar | 1932 before ROC 民前1932年 |
| Nanakshahi calendar | −1488 |
| Seleucid era | 291/292 AG |
| Thai solar calendar | 522–523 |
| Tibetan calendar | 阴土猪年 (female Earth-Pig) 106 or −275 or −1047 — to — 阳金鼠年 (male Iron-Rat) 107 or −274 or −1046 |
Year 21 BC was either a common year starting on Monday, Tuesday or Wednesday or a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) and a leap year starting on Sunday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Lollius and Lepidus (or, less frequently, year 733 Ab urbe condita ). The denomination 21 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
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The Julian calendar, proposed by Julius Caesar in AUC 708, was a reform of the Roman calendar. It took effect on 1 January AUC 709 , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandria.
Year 27 BC was either a common year starting on Sunday, Monday or Tuesday or a leap year starting on Monday of the Julian calendar and a common year starting on Sunday of the Proleptic Julian calendar. At the time, it was known as the Year of the Second Consulship of Octavian and Agrippa. The denomination 27 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
AD 4 was a common year starting on Wednesday or a leap year starting on Tuesday of the Julian calendar and a leap year starting on Tuesday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Catus and Saturninus. The denomination "AD 4" for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 4 BC was a common year starting on Tuesday or Wednesday of the Julian calendar and a common year starting on Monday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Sabinus and Rufus. The denomination 4 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 5 BC was a common year starting on Monday or Tuesday of the Julian calendar and a leap year starting on Saturday of the Proleptic Julian calendar. In the Roman world, it was known as the Year of the Consulship of Augustus and Sulla. The denomination 5 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 10 BC was either a common year starting on Tuesday, Wednesday or Thursday or a leap year starting on Tuesday or Wednesday of the Julian calendar and a common year starting on Sunday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Maximus and Antonius. The denomination 10 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 11 BC was either a common year starting on Monday or Tuesday or a leap year starting on Sunday, Monday or Tuesday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Tubero and Maximus. The denomination 11 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 37 BC was either a common year starting on Monday, Tuesday or Wednesday or a leap year starting on Monday or Tuesday of the Julian calendar and a leap year starting on Monday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Agrippa and Gallus. The denomination 37 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 41 BC was either a common year starting on Wednesday or Thursday or a leap year starting on Tuesday, Wednesday or Thursday of the Julian calendar and a leap year starting on Wednesday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Antonius and Vatia. The denomination 41 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 25 BC was either a common year starting on Wednesday, Thursday or Friday or a leap year starting on Wednesday or Thursday of the Julian calendar and a leap year starting on Tuesday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Silanus. The denomination 25 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 26BC was either a common year starting on Tuesday or Wednesday or a leap year starting on Monday, Tuesday or Wednesday of the Julian calendar and a common year starting on Monday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Taurus. The denomination 26 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 20 BC was either a common year starting on Wednesday or Thursday or a leap year starting on Tuesday, Wednesday or Thursday of the Julian calendar and a common year starting on Tuesday of the Proleptic Julian calendar. At the time, it was also known as the Year of the Consulship of Appuleius and Nerva. The denomination 20 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 16 BC was either a common year starting on Monday, Tuesday or Wednesday or a leap year starting on Monday or Tuesday of the Julian calendar and a common year starting on Sunday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Ahenobarbus and Scipio. The denomination 16 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
Year 22 BC was either a common year starting on Sunday, Monday or Tuesday or a leap year starting on Sunday or Saturday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Marcellus and Arruntius. The denomination 22 BC for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
A common year starting on Tuesday is any non-leap year that begins on Tuesday, 1 January, and ends on Tuesday, 31 December. Its dominical letter hence is F. The most recent year of such kind was 2019 and the next one will be 2030, or, likewise, 2014 and 2025 in the obsolete Julian calendar, see below for more.
A leap year starting on Tuesday is any year with 366 days that begins on Tuesday, 1 January, and ends on Wednesday, 31 December. Its dominical letters hence are FE. The most recent year of such kind was 2008 and the next one will be 2036 in the Gregorian calendar or, likewise 2020 and 2048 in the obsolete Julian calendar.
A leap year starting on Saturday is any year with 366 days that begins on Saturday, 1 January, and ends on Sunday, 31 December. Its dominical letters hence are BA. The most recent year of such kind was 2000 and the next one will be 2028 in the Gregorian calendar or, likewise, 2012 and 2040 in the obsolescent Julian calendar. In the Gregorian calendar, years divisible by 400 are always leap years starting on Saturday. The most recent such year was 2000 and the next one will be 2400, see below for more.
The determination of the day of the week for any date may be performed with a variety of algorithms. In addition, perpetual calendars require no calculation by the user, and are essentially lookup tables. A typical application is to calculate the day of the week on which someone was born or a specific event occurred.
The Gregorian calendar is the calendar used in most of the world. It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar, reducing the average year from 365.25 days to 365.2425 days, and adjusting for the drift in the 'tropical' or 'solar' year that the inaccuracy had caused during the intervening centuries.
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to the dates preceding its official introduction in 1582. In nations that adopted the Gregorian calendar after its official and first introduction, dates occurring in the interim period of 15 October 1582 to the date on which the pertinent nation adopted the Gregorian calendar and abandoned the Julian calendar are sometimes 'Gregorianized' also. For example, the birthday of President George Washington was originally dated 11 February 1731 because Great Britain, of which he was born a subject, used the Julian calendar and dated the beginning of English years as 25 March. After Great Britain switched to the Gregorian calendar, Washington's birthday was dated 22 February 1732 proleptically, according to the Gregorian calendar applied backward. This remains the modern dating of his birthday.