| 104461 | September 12 | Death | 2014 – Hugh Royer, Jr., American golfer (b. 1936) |
| 104462 | September 12 | Death | 2015 – Claudia Card, American philosopher and academic (b. 1940) |
| 104463 | September 12 | Death | 2015 – Frank D. Gilroy, American playwright and screenwriter (b. 1925) |
| 104464 | September 12 | Death | 2015 – Al Monchak, American baseball player and coach (b. 1917) |
| 104465 | September 12 | Death | 2015 – Aronda Nyakairima, Ugandan general and politician (b. 1959) |
| 104466 | September 12 | Holiday and observance | Christian feast day:Ailbe of EmlyGuy of AnderlechtHoly Name of MaryJohn Henry Hobart (Episcopal Church (USA))Laisrén mac Nad FroíchSacerdos of LyonSeptember 12 (Eastern Orthodox liturgics) |
| 104467 | September 12 | Holiday and observance | Earliest date on which Programmers' Day can fall, while September 13 is the latest celebrated on the 256th day of the year (Russia and programmers around the world) |
| 104468 | September 12 | Holiday and observance | Day of Conception (Russia) |
| 104469 | September 12 | Holiday and observance | Defenders Day (Maryland. (United States) |
| 104470 | September 12 | Holiday and observance | Enkutatash falls on this day if it is a leap year. Celebrated on the first day of Mäskäräm. (Ethiopia) |
| 104471 | September 12 | Holiday and observance | Mindfulness Day |
| 104472 | September 12 | Holiday and observance | National Chocolate Milkshake Day (United States) |
| 104473 | September 12 | Holiday and observance | National Day (Cape Verde) |
| 104474 | September 12 | Holiday and observance | National Day of Encouragement (United States) |
| 104475 | September 12 | Holiday and observance | Saragarhi Day (Sikhism) (this day or nearest weekday) |
| 104476 | September 13 | Event | 585 BC – Lucius Tarquinius Priscus, king of Rome, celebrates a triumph for his victories over the Sabines, and the surrender of Collatia. |
| 104477 | September 13 | Event | 509 BC – The Temple of Jupiter Optimus Maximus on Rome's Capitoline Hill is dedicated on the ides of September. |
| 104478 | September 13 | Event | 379 – Yax Nuun Ayiin I is coronated as 15 Ajaw of Tikal |
| 104479 | September 13 | Event | 533 – Belisarius of the Byzantine Empire defeats Gelimer and the Vandals at the Battle of Ad Decimum, near Carthage, North Africa. |
| 104480 | September 13 | Event | 1229 – Ögedei Khan is proclaimed Khagan of the Mongol Empire in Kodoe Aral, Khentii: Mongolia. |
| 104481 | September 13 | Event | 1437 – Battle of Tangier: a Portuguese expeditionary force initiates a failed attempt to seize the Moroccan citadel of Tangier. |
| 104482 | September 13 | Event | 1501 – Michelangelo begins work on his statue of David. |
| 104483 | September 13 | Event | 1504 – Queen Isabella and King Ferdinand issue a Royal Warrant for the construction of a Royal Chapel (Capilla Real) to be built. |
| 104484 | September 13 | Event | 1541 – After three years of exile, John Calvin returns to Geneva to reform the church under a body of doctrine known as Calvinism. |
| 104485 | September 13 | Event | 1584 – San Lorenzo del Escorial Palace in Madrid is finished. |
| 104486 | September 13 | Event | 1609 – Henry Hudson reaches the river that would later be named after him – the Hudson River. |
| 104487 | September 13 | Event | 1645 – Battle of Philiphaugh Covenanters win the day over the royalists. |
| 104488 | September 13 | Event | 1743 – Great Britain, Austria and the Kingdom of Sardinia sign the Treaty of Worms. |
| 104489 | September 13 | Event | 1759 – Battle of the Plains of Abraham: the British defeat the French near Quebec City in the Seven Years' War, known in the United States as the French and Indian War. |
| 104490 | September 13 | Event | 1782 – American Revolutionary War: Franco-Spanish troops launch the unsuccessful "grand assault" during the Great Siege of Gibraltar. |