Byzantine Emperor Justin I names his nephew Justinian I as co-ruler and successor to the throne.

The foundation of Constantinople in 330 AD marks the conventional start of the Eastern Roman Empire, which fell to the Ottoman Empire in 1453 AD. Only the emperors who were recognized as legitimate rulers and exercised sovereign authority are included, to the exclusion of junior co-emperors who never attained the status of sole or senior ruler, as well as of the various usurpers or rebels who claimed the imperial title.

Justin I, also called Justin the Thracian, was Roman emperor from 518 to 527. Born to a peasant family, he rose through the ranks of the army to become commander of the imperial guard and when Emperor Anastasius I Dicorus died, he out-maneouvered his rivals and was elected as his successor, in spite of being around 68 years old. His reign is significant for the founding of the Justinian dynasty that included his nephew, Justinian I, and three succeeding emperors. His consort was Empress Euphemia.

Justinian I, also known as Justinian the Great, was Byzantine Roman emperor from 527 to 565.

April 1 is the 91st day of the year in the Gregorian calendar; 274 days remain until the end of the year.