| Millennium: | 4th Millennium BC |
|---|---|
| Centuries: | 41st cent. BC – 40th cent. BC – 39th cent. BC |
| Decades: | 4020s BC 4010s BC 4000s BC – 3990s BC – 3980s BC 3970s BC 3960s BC |
| Years: | 3994 BC 3993 BC 3992 BC – 3991 BC – 3990 BC 3989 BC 3988 BC |
| Gregorian calendar | 3991 MMMCMXCI |
| Ab urbe condita | 4744 |
| Armenian calendar | 3440 ԹՎ ՎՆԽ |
| Assyrian calendar | 8741 |
| Bahá'í calendar | 2147–2148 |
| Bengali calendar | 3398 |
| Berber calendar | 4941 |
| British Regnal year | N/A |
| Buddhist calendar | 4535 |
| Burmese calendar | 3353 |
| Byzantine calendar | 9499–9500 |
| Chinese calendar | 庚寅年 (Metal Tiger) 6687 or 6627 — to — 辛卯年 (Metal Rabbit) 6688 or 6628 |
| Coptic calendar | 3707–3708 |
| Discordian calendar | 5157 |
| Ethiopian calendar | 3983–3984 |
| Hebrew calendar | 7751–7752 |
| Hindu calendars | |
| - Vikram Samvat | 4047–4048 |
| - Shaka Samvat | 3912–3913 |
| - Kali Yuga | 7091–7092 |
| Holocene calendar | 13991 |
| Igbo calendar | 2991–2992 |
| Iranian calendar | 3369–3370 |
| Islamic calendar | 3472–3473 |
| Japanese calendar | Heisei 2003 (平成２００３年) |
| Javanese calendar | 3984–3985 |
| Juche calendar | 2080 |
| Julian calendar | Gregorian minus 28 days |
| Korean calendar | 6324 |
| Minguo calendar | ROC 2080 民國2080年 |
| Nanakshahi calendar | 2523 |
| Thai solar calendar | 4534 |
| Unix time | 63776592000–63808127999 |
The year 3991 BC