Year 20 BC was either a common year starting on WednesdayorThursday or a leap year starting on Tuesday, WednesdayorThursday (link will display the full calendar) of the Julian calendar (the sources differ, see leap year error for further information) 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 (or, less frequently, year 734 Ab urbe condita). 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.
| Millennium: | 1st millennium BC |
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| Centuries: |
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| Years: |
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| Gregorian calendar | 20 BC XX BC |
| Ab urbe condita | 734 |
| Ancient Greek era | 190th Olympiad (victor)¹ |
| Assyrian calendar | 4731 |
| Balinese saka calendar | N/A |
| Bengali calendar | −612 |
| Berber calendar | 931 |
| Buddhist calendar | 525 |
| Burmese calendar | −657 |
| Byzantine calendar | 5489–5490 |
| Chinese calendar | 庚子年 (Metal Rat) 2678 or 2471 — to — 辛丑年 (Metal Ox) 2679 or 2472 |
| Coptic calendar | −303 – −302 |
| Discordian calendar | 1147 |
| Ethiopian calendar | −27 – −26 |
| Hebrew calendar | 3741–3742 |
| Hindu calendars | |
| - Vikram Samvat | 37–38 |
| - Shaka Samvat | N/A |
| - Kali Yuga | 3081–3082 |
| Holocene calendar | 9981 |
| Iranian calendar | 641 BP – 640 BP |
| Islamic calendar | 661 BH – 660 BH |
| Javanese calendar | N/A |
| Julian calendar | 20 BC XX BC |
| Korean calendar | 2314 |
| Minguo calendar | 1931 before ROC 民前1931年 |
| Nanakshahi calendar | −1487 |
| Seleucid era | 292/293 AG |
| Thai solar calendar | 523–524 |
| Tibetan calendar | 阳金鼠年 (male Iron-Rat) 107 or −274 or −1046 — to — 阴金牛年 (female Iron-Ox) 108 or −273 or −1045 |