How does one determine the date a constructed language was invented?

Question

Some constructed languages list a date, or at least year, of creation. Is there any official or unofficial standard to determine that date for any given conlang? The way I see it, there are several possibilities, at least the ones I thought up:

• The date that work began on the language
• The date of the first evidence of work on the language, if the prior date is not known
• The date of the first usage of the language (this could be broken down into public or private usage)
• The date of the first publication that mentions the language
• The publication date of the first dictionary and/or grammar

Answer 1

For auxiliary languages it's usually publication date of the first grammar/dictionary released to the public or the first work dedicated to the language, whichever comes first.

For Esperanto this was Unua Libro, which was published in 1887 - more than a year after Zamenhof began working on the language. Wikipedia also lists the year of creation of Occidental as 1922, the year de Wahl published the first edition of the Kosmoglotta magazine, despite there being evidence that the language was used much before then, both in private conversation (letters etc.) and even a publication written entirely in Occidental.

The "date of creation" for other types of conlangs seems to be less well-defined, although I'd expect that fictional languages would be associated with the publication of the first book/movie/game that uses them (perhaps with the exception of Tolkien's languages). Wikipedia also uses the day Lojban began being developed as its creation date.