| Millennium: | 1st millennium |
|---|---|
| Centuries: | |
| Decades: | |
| Years: |
| Gregorian calendar | 840 DCCCXL |
| Ab urbe condita | 1593 |
| Armenian calendar | 289 ԹՎ ՄՁԹ |
| Assyrian calendar | 5590 |
| Bengali calendar | 247 |
| Berber calendar | 1790 |
| Buddhist calendar | 1384 |
| Burmese calendar | 202 |
| Byzantine calendar | 6348–6349 |
| Chinese calendar |
己未年 (Earth Goat) 3536 or 3476 — to — 庚申年 (Metal Monkey) 3537 or 3477 |
| 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 |
Year 840 (DCCCXL) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar.