| Millennium: | 1st millennium BC |
|---|---|
| Centuries: | 2nd century BC – 1st century BC – 1st century |
| Decades: | 50s BC 40s BC 30s BC – 20s BC – 10s BC 0s BC 0s |
| Years: | 24 BC 23 BC 22 BC – 21 BC – 20 BC 19 BC 18 BC |
| 21 BC by topic | |
| Politics | |
| State leaders – Sovereign states | |
| Birth and death categories | |
| Births – Deaths | |
| Establishments and disestablishments categories | |
| Establishments – Disestablishments | |
| Gregorian calendar | 21 BC |
| Ab urbe condita | 733 |
| Armenian calendar | N/A |
| Bahá'í calendar | -1864 – -1863 |
| Bengali calendar | -613 |
| Berber calendar | 930 |
| Buddhist calendar | 524 |
| Burmese calendar | -658 |
| Byzantine calendar | 5488 – 5489 |
| Chinese calendar | 己亥年 (2616/2676) — to —
庚子年(2617/2677) |
| Coptic calendar | -304 – -303 |
| Ethiopian calendar | -28 – -27 |
| Hebrew calendar | 3740 – 3741 |
| Hindu calendars | |
| - Bikram Samwat | 36 – 37 |
| - Shaka Samvat | N/A |
| - Kali Yuga | 3081 – 3082 |
| Holocene calendar | 9980 |
| Iranian calendar | 642 BP – 641 BP |
| Islamic calendar | 662 BH – 661 BH |
| Japanese calendar | |
| Korean calendar | 2313 |
| Thai solar calendar | 523 |
Year 21 BC was a leap year starting on Sunday (link will display the full calendar) of the Julian calendar.