erayoudaoicibaDictYouDict[era 词源字典]
era: [17] In ancient Rome, small discs or tokens made of ‘brass’ (Latin aes, a descendant, like English ore [OE], of Indo-European *ajes) used for counting were known as area. In due course this developed the metaphorical meaning ‘number as a basis for calculation’, and from around the 5th century AD it came to be used in Spain, North Africa, and southern Gaul as a prefix for dates, some what analogous to modern English AD.

By extension it was then applied to a ‘system of chronological notation, as dated from a particular event or point in time’, the sense in which English acquired the word. The more general ‘historical period’ is an 18thcentury semantic development.
=> ore[era etymology, era origin, 英语词源]
era (n.)youdaoicibaDictYouDict
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.