| Millennium: | 1st millennium |
|---|---|
| Centuries: | |
| Decades: | |
| Years: |
| 840 by topic |
|---|
| Leaders |
| Categories |
| Gregorian calendar | 840 DCCCXL |
| Ab urbe condita | 1593 |
| Armenian calendar | 289 ԹՎ ՄՁԹ |
| Assyrian calendar | 5590 |
| Balinese saka calendar | 761–762 |
| Bengali calendar | 247 |
| Berber calendar | 1790 |
| Buddhist calendar | 1384 |
| Burmese calendar | 202 |
| Byzantine calendar | 6348–6349 |
| Chinese calendar | 己未年 (Earth Goat) 3537 or 3330 — to — 庚申年 (Metal Monkey) 3538 or 3331 |
| Coptic calendar | 556–557 |
| Discordian calendar | 2006 |
| Ethiopian calendar | 832–833 |
| Hebrew calendar | 4600–4601 |
| Hindu calendars | |
| - Vikram Samvat | 896–897 |
| - Shaka Samvat | 761–762 |
| - Kali Yuga | 3940–3941 |
| Holocene calendar | 10840 |
| Iranian calendar | 218–219 |
| Islamic calendar | 225–226 |
| Japanese calendar | Jōwa 7 (承和７年) |
| Javanese calendar | 737–738 |
| Julian calendar | 840 DCCCXL |
| Korean calendar | 3173 |
| Minguo calendar | 1072 before ROC 民前1072年 |
| Nanakshahi calendar | −628 |
| Seleucid era | 1151/1152 AG |
| Thai solar calendar | 1382–1383 |
| Tibetan calendar | 阴土羊年 (female Earth-Goat) 966 or 585 or −187 — to — 阳金猴年 (male Iron-Monkey) 967 or 586 or −186 |
Year 840 (DCCCXL) was a leap year starting on Thursday in the Julian calendar, the 840th year of the Common Era (CE) and Anno Domini (AD) designations, the 840th year of the 1st millennium, the 40th year of the 9th century, and the 1st year of the 840s decade.