| Centuries: | 6th century · 7th century · 8th century |
| Decades: | 600s 610s 620s 630s 640s 650s 660s |
| Years: | 635 636 637 638 639 640 641 |
| 638 by topic | |
| Politics | |
| State leaders – Sovereign states | |
| Birth and death categories | |
| Births – Deaths | |
| Establishment and disestablishment categories | |
| Establishments – Disestablishments | |
| Gregorian calendar | 638 DCXXXVIII |
| Ab urbe condita | 1391 |
| Armenian calendar | 87 ԹՎ ՁԷ |
| Bahá'í calendar | -1206 – -1205 |
| Bengali calendar | 45 |
| Berber calendar | 1588 |
| Buddhist calendar | 1182 |
| Burmese calendar | 0 |
| Byzantine calendar | 6146 – 6147 |
| Chinese calendar | 丁酉年十二月十一日 (3274/3334-12-11) — to —
戊戌年十一月廿一日(3275/3335-11-21) |
| Coptic calendar | 354 – 355 |
| Ethiopian calendar | 630 – 631 |
| Hebrew calendar | 4398 – 4399 |
| Hindu calendars | |
| - Bikram Samwat | 694 – 695 |
| - Shaka Samvat | 560 – 561 |
| - Kali Yuga | 3739 – 3740 |
| Holocene calendar | 10638 |
| Iranian calendar | 16 – 17 |
| Islamic calendar | 16 – 17 |
| Japanese calendar | |
| Korean calendar | 2971 |
| Thai solar calendar | 1181 |