| Millennium: | 4th Millennium BC |
|---|---|
| Centuries: | 39th cent. BC – 38th cent. BC – 37th cent. BC |
| Decades: | 3750s BC 3740s BC 3730s BC – 3720s BC – 3710s BC 3700s BC 3690s BC |
| Years: | 3724 BC 3723 BC 3722 BC – 3721 BC – 3720 BC 3719 BC 3718 BC |
| Gregorian calendar | 3721 MMMDCCXXI |
| Ab urbe condita | 4474 |
| Armenian calendar | 3170 ԹՎ ՎՃՀ |
| Assyrian calendar | 8471 |
| Bahá'í calendar | 1877–1878 |
| Bengali calendar | 3128 |
| Berber calendar | 4671 |
| British Regnal year | N/A |
| Buddhist calendar | 4265 |
| Burmese calendar | 3083 |
| Byzantine calendar | 9229–9230 |
| Chinese calendar | 庚申年 (Metal Monkey) 6417 or 6357 — to — 辛酉年 (Metal Rooster) 6418 or 6358 |
| Coptic calendar | 3437–3438 |
| Discordian calendar | 4887 |
| Ethiopian calendar | 3713–3714 |
| Hebrew calendar | 7481–7482 |
| Hindu calendars | |
| - Vikram Samvat | 3777–3778 |
| - Shaka Samvat | 3642–3643 |
| - Kali Yuga | 6821–6822 |
| Holocene calendar | 13721 |
| Igbo calendar | 2721–2722 |
| Iranian calendar | 3099–3100 |
| Islamic calendar | 3194–3195 |
| Japanese calendar | Heisei 1733 (平成１７３３年) |
| Javanese calendar | 3706–3707 |
| Juche calendar | 1810 |
| Julian calendar | Gregorian minus 26 days |
| Korean calendar | 6054 |
| Minguo calendar | ROC 1810 民國1810年 |
| Nanakshahi calendar | 2253 |
| Thai solar calendar | 4264 |
| Unix time | 55256256000–55287791999 |
The year 3721 BC