Decades: 370s 380s 390s 400s 410s - 420s - 430s 440s 450s 460s 470s

Years: 420 421 422 423 424 - 425 - 426 427 428 429 430

• October 23 -Valentinian III becomes western Roman emperor.
• Aetius leads an army into Italy to support the self-proclaimed emperor Joannes.
• Last known usage of Demotic.
• The Sanhedrin is disbanded by the Roman Empire.
• Buddhism begins to spread to Southeast Asia.

• Zu Chongzhi, Chinese mathematician

• October 10 - Atticus, Patriarch of Constantinople
• Joannes, usurper in the western Roman Empire.