Diocletian ( Gaius Aurelius Valerius Diocletianus , ~ 245 to ~ 312 ) was Roman Emperor from 20 November 284 to 11 May 305 . Diocletian ended the period known as the Crisis of the Third Century ( 235 â `` 284 ) . Gaius Aurelius Valerius Diocletianus ( c. 22 December 244 â `` 3 December 311 ) , commonly known as Diocletian , was a Roman Emperor from 284 to 305 .