era (n.)

1716, earlier aera (1610s), from Late Latin aera, era "an era or epoch from which time is reckoned" (7c.), probably identical with Latin aera "counters used for calculation," plural of aes (genitive aeris) "brass, copper, money" (see ore, also compare copper). The Latin word's use in chronology said to have begun in 5c. Spain (where the local era, aera Hispanica, began 38 B.C.E.; some say because of a tax levied that year). Other ancient eras included the Chaldean (autumn of 311 B.C.E.), the Era of Actium (31 B.C.E.), of Antioch (49 B.C.E.), of Tyre (126 B.C.E.), the Olympiadic (July 1, 776 B.C.E.) and the Seleucidan (autumn 312 B.C.E.). In English it originally meant "the starting point of an age" (compare epoch); meaning "system of chronological notation" is from 1640s; that of "historical period" is from 1741, as in the U.S. Era of Good Feeling (1817) was anything but.