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 XXI 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) 2677 or 2470 — to — 庚子年 (Metal Rat) 2678 or 2471 |
| 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 XXI 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 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 is a solar calendar of 365 days in every year with an additional leap day every fourth year. The Julian calendar is still used as a religious calendar in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Amazigh people.
A leap year is a calendar year that contains an additional day compared to a common year. The 366th day is added to keep the calendar year synchronised with the astronomical year or seasonal year. Since astronomical events and seasons do not repeat in a whole number of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day—a leap day—or month—a leap month—into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected.
Year 31 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 Tuesday of the Proleptic Julian calendar. At the time, it was known as the Year of the Consulship of Antonius and Caesar or as Caesar and Messalla. The denomination 31 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 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. In the Roman Empire, 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.
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 26 BC 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.
Year 15 BC was either a common year starting on Tuesday, Wednesday or Thursday or a leap year starting on 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 Drusus and Piso. The denomination 15 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 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.
The Gregorian calendar is the calendar used in most parts of the world. It went into effect in October 1582 following the papal bull Inter gravissimas issued by Pope Gregory XIII, which introduced it as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day "tropical" or "solar" year that is determined by the Earth's revolution around the Sun.
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 U.S. 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 instead of 1 January. 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.