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|309 by topic|
|Ab urbe condita||1062|
|Balinese saka calendar||230–231|
|Chinese calendar||戊辰年 (Earth Dragon)|
3005 or 2945
— to —
己巳年 (Earth Snake)
3006 or 2946
|- Vikram Samvat||365–366|
|- Shaka Samvat||230–231|
|- Kali Yuga||3409–3410|
|Iranian calendar||313 BP – 312 BP|
|Islamic calendar||323 BH – 322 BH|
|Minguo calendar||1603 before ROC|
|Seleucid era||620/621 AG|
|Thai solar calendar||851–852|
435 or 54 or −718
— to —
436 or 55 or −717
Year 309 (CCCIX) was a common 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 Licinianus and Constantius (or, less frequently, year 1062 Ab urbe condita). The denomination 309 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.
- The Spanish provinces revolt from the control of Maxentius, acknowledging Constantine the Great as their Emperor.
- A plague that may be related to anthrax spreads across the Roman Empire, causing a drastic decline in the population.
- King Hormizd II, Shah of the Persian Empire, dies after a 7-year reign and a power struggle for the succession ensues. He is succeeded by his son Shapur II.
- Pope Marcellus I is banished from Rome by the Emperor Maxentius.
- April 18 – Pope Eusebius succeeds Marcellus as the 31st pope, but is himself banished on August 17 to Sicily (These events may have taken place in 310).
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- Hormizd II, king of the Persian Empire
- Pope Marcellus I
- Saint Alban (possible date—also 304)
- Saint Elias and companions