| Millennium: | 1st millennium |
|---|---|
| Centuries: | 1st century BC – 1st century – 2nd century |
| Decades: | 0s BC 0s 10s – 20s – 30s 40s 50s |
| Years: | 18 19 20 – 21 – 22 23 24 |
| 21 by topic | |
| Politics | |
| State leaders – Sovereign states | |
| Birth and death categories | |
| Births – Deaths | |
| Establishment and disestablishment categories | |
| Establishments – Disestablishments | |
| Gregorian calendar | 21 XXI |
| Ab urbe condita | 774 |
| Armenian calendar | N/A |
| Bahá'í calendar | -1823 – -1822 |
| Bengali calendar | -572 |
| Berber calendar | 971 |
| Buddhist calendar | 565 |
| Burmese calendar | -617 |
| Byzantine calendar | 5529 – 5530 |
| Chinese calendar | 庚辰年十二月廿九日 (2657/2717-12-29) — to —
辛巳年十二月初九日(2658/2718-12-9) |
| Coptic calendar | -263 – -262 |
| Ethiopian calendar | 13 – 14 |
| Hebrew calendar | 3781 – 3782 |
| Hindu calendars | |
| - Bikram Samwat | 77 – 78 |
| - Shaka Samvat | N/A |
| - Kali Yuga | 3122 – 3123 |
| Holocene calendar | 10021 |
| Iranian calendar | 601 BP – 600 BP |
| Islamic calendar | 619 BH – 618 BH |
| Japanese calendar | |
| Korean calendar | 2354 |
| Thai solar calendar | 564 |
Year 21 (XXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar.