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|Gregorian calendar||AD 21
|Ab urbe condita||774|
|Chinese calendar||庚辰年 (Metal Dragon)
2717 or 2657
— to —
辛巳年 (Metal Snake)
2718 or 2658
|Coptic calendar||−263 – −262|
|- Vikram Samvat||77–78|
|- Shaka Samvat||N/A|
|- Kali Yuga||3121–3122|
|Iranian calendar||601 BP – 600 BP|
|Islamic calendar||619 BH – 618 BH|
|Julian calendar||AD 21
|Minguo calendar||1891 before ROC
|Seleucid era||332/333 AG|
|Thai solar calendar||563–564|
AD 21 (XXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Caesar (or, less frequently, year 774 Ab urbe condita). The denomination AD 21 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.
- Revolt of the Aedui under Julius Florus and Julius Sacrovir, suppressed by Gaius Silius.
- Emperor Tiberius is a Roman Consul for the fourth time.
- The Romans create a buffer state in the territory of the Quadi in southern Slovakia.
- Barracks are constructed for the Praetorian Guard on the Quirinal.
- King Daeso of Dongbuyeo is killed in battle against the armies of Goguryeo, led by its third ruler, King Daemusin.
Arts and sciences
- The manufacture of pens and metal writing tools begins in Rome.
- Publius Sulpicius Quirinius, Roman governor of Syria (b. c. 51 BC)
- Marcus Valerius Messalla Barbatus, Roman Consul (or AD 20) (b. 11 BC)
- Arminius, Germanic leader who defeated three Roman legions at the Battle of the Teutoburg Forest (b. 18/17 BC)
- Empress Wang
- King Daeso of Dongbuyeo (b. 60 BC)