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Year 312 (CCCXII) was a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantinus and Licinianus (or, less frequently, year 1065 Ab urbe condita). The denomination 312 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. 312 (CCCXII) was a leap year starting on Tuesday of the Julian calendar, the 312th year of the Common Era (CE) and Anno Domini (AD) designations, the 312th year of the 1st millennium, the 12th year of the 4th century, and the 3rd year of the 310s decade. As of the start of 312, the Gregorian calendar was 1 day ahead of the Julian calendar, which was the dominant calendar of the time.
|Ab urbe condita||1065|
|Balinese saka calendar||233–234|
|Chinese calendar||辛未年 (Metal Goat)|
3008 or 2948
— to —
壬申年 (Water Monkey)
3009 or 2949
|- Vikram Samvat||368–369|
|- Shaka Samvat||233–234|
|- Kali Yuga||3412–3413|
|Iranian calendar||310 BP – 309 BP|
|Islamic calendar||320 BH – 319 BH|
|Minguo calendar||1600 before ROC|
|Seleucid era||623/624 AG|
|Thai solar calendar||854–855|
438 or 57 or −715
— to —
439 or 58 or −714
- Constantine the Great crosses the Cottian Alps with an army (40,000 men) and auxiliaries. He defeats Maxentius in three battles at Turin, Brescia and Verona. Maxentius's most senior commander Ruricius Pompeianus is killed during the fightings.
- October 28 – Battle of the Milvian Bridge: Constantine I defeats usurper Maxentius at the Milvian Bridge and becomes the only Roman emperor in the West. During the battle, he reportedly has a vision of a cross (labarum) with the phrase "in hoc signo vinces" ("In this sign you shall conquer").
- October 29 – Constantine I enters Rome; he stages a grand adventus in the city, and is met with popular jubilation. Maxentius' body is fished out of the Tiber and decapitated.
- Constantine I forges an alliance with co-emperor Licinius, and offers him his halfsister Constantia in marriage. The Praetorian Guard and Imperial Horse Guard (equites singulares Augusti) are disbanded.
- Construction begins on the Arch of Constantine in Rome. On a sculptural relief Constantine I addresses the people in the Roman Forum (approximate date).
- Constantine I adopts the words "in hoc signo vinces" as a motto and have the letters X and P (the first letters of the Greek word Christ) emblazoned on the shields of his soldiers.
- The Council of Carthage supports Donatism, which espouses a rigorous application and interpretation of the sacraments. These doctrines will be condemned by the Council of Arles (314).
- Constantine I promotes a policy of state sponsorship of Christianity, perhaps even becoming a Christian himself (see Constantine the Great and Christianity).