I've been taking notes for just a few weeks, and it's still very much a beginner's pile of vocabulary and grammar notes. Though we did talk in detail about one topic: era names (元号).
An old textbook I have referred to Japanese years using 元号, such as 平成六年 for the 6th year of the Heisei Era, or 1994. In the wild, though, we see a lot of plain 4-digit decimal years; some institutions, like NHK news, provide both, such as: "2015年 (平成27年)". I asked a native speaker at our conversation group: how commonly used are 元号 years outside of textbooks?
His answer: most people, in casual situations, use decimal years; but official documents, both corporate and governmental, tend to use 元号. Doing so honors a tradition dating back more than 1,300 years, and brings up some interesting points regarding the Japanese Emperor.
In Japan the current Emperor's given name (in 2015, Akihito) is not used. To refer to him, instead the term 天皇陛下 (his Majesty the Emperor) is used; or more formally, 今上天皇 (the current Emperor). He acceded to the throne in 1989 and began the Heisei Era (平成時代), starting with 平成1年 (there is no year zero). The term 元年 (origin year) is also used for year 1 of an era.
By custom, at Akihito's death, he will be given the posthumous name 平成天皇 (Emperor Heisei) and the 平成時代 will come to an end. In other words, the name of his reigning era is set aside for him. This last happened in 1989 when Akihito's father, Hirohito, was named Showa (昭和) as his eponymous era (1926-1989) was brought to a close.
Only at the start of Akihito's reign was the 平成 name unveiled. By custom 元号 names are a pair of kanji with positive connotations of tradition, peace, brightness, and so on. 昭和 is literally "shining" + "harmony/Japan". 平成 is "peace" + "become". The names must also be easy to read and write, and not duplicate another era or an existing Japanese word.
In 2016, however, Akihito announced a plan to abdicate the throne on January 1, 2018 in favor of his crown prince son Naruhito. This last happened 200 years ago and brings up a number of questions, including handling of the era changeover. Businesses and makers of calendars are asking for advance notice of the new era name. As for the 平成天皇 name, would that be given to Akihito when he steps down, and the 平成 era ends, or only after his death?
The current Emperor is the 125th to take the throne, and continues a string of 元号 eras dating back to 701 CE (大宝 era). The tradition of One Era, One Emperor, however, is much newer, starting in the Meiji (明治) Era in 1868. Before that, more than one era was associated with a single Emperor (and sometimes vice versa). As a curiosity, beginning and ending years may be less than 365 days long, as an era starts on the date the Emperor takes the throne, while year numbers increment on January 1.
(How does a 2015 article refer to a 2016 new story? I'm porting these old 2015 docs to web pages in 2017, and adding new information.)
お疲れ様, thanks for reading!