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Year 304 (CCCIV) was a leap year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Valerius and Valerius (or, less frequently, year 1057 Ab urbe condita). The denomination 304 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
|Ab urbe condita||1057|
|Balinese saka calendar||225–226|
|Chinese calendar||癸亥年 (Water Pig)|
3000 or 2940
— to —
甲子年 (Wood Rat)
3001 or 2941
|- Vikram Samvat||360–361|
|- Shaka Samvat||225–226|
|- Kali Yuga||3404–3405|
|Iranian calendar||318 BP – 317 BP|
|Islamic calendar||328 BH – 327 BH|
|Minguo calendar||1608 before ROC|
|Seleucid era||615/616 AG|
|Thai solar calendar||846–847|
430 or 49 or −723
— to —
431 or 50 or −722
- Emperor Diocletian issues four edicts aimed at destroying Christianity; churches are to be dismantled, clergymen arrested, and their followers forced to sacrifice to pagan gods on pain of death. Those put to death include Agnes of Rome, a 12-year-old Christian girl who has refused marriage and consecrated her virginity to God. Hailed as a martyr, she will be honored as the patron saint of chastity, gardeners, rape victims and virgins.
- Constantius Chlorus defeats the Alemanni and fortifies the town of Konstanz (Germany).
- Diocletian fights a campaign against the Carpi on the Danube and becomes seriously ill.
- A Triumphal Arch is built for Diocletian in Rome.
- Sixteen Kingdoms: The Wu Hu uprising establishes the Han Kingdom, under Liu Yuan.
- Sichuan earns its independence from China.
- Biryu becomes king of the Korean kingdom of Baekje.
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- October 25 – Pope Marcellinus
- December 25 – Saint Anastasia (martyred)
- Date unknown
- Saint Afra (martyred by fire)
- Saint Agape, Chionia, and Irene (martyred)
- Saint Agnes (martyred)
- Saint Alban (possibly 309)
- Bunseo of Baekje, king of Baekje (Korea)
- Saint Florian (martyred)
- Saint Gorgonius of Nicomedia (martyred)
- Saint Juliana of Nicomedia (martyred)
- Saint Lucie of Syracuse (martyred)
- Saint Margaret (martyred)
- Saint Pancras (martyred)
- Saint Philomena (martyred)
- Sima Ai, Chinese prince of the Jin dynasty (b. 277)
- Saints Theodora and Didymus (martyred)
- Saint Vincent of Saragossa (martyred)
- "List of Rulers of Korea". www.metmuseum.org. Retrieved April 20, 2019.
- Knechtges, David R.; Chang, Taiping (2010). Ancient and Early Medieval Chinese Literature (vol.I): A Reference Guide, Part One. BRILL. p. 542. ISBN 9789004191273.