When is the event date in parentheses?

rraymond:

As a rule, punctuation in citations follow the same principles as punctuation in any type of writing. Parentheses are generally used in two situations:

1. To add details that are not essential to the sense of the sentence but still contribute to an understanding of the subject;
2. To enclose numbers or letters that are used to enumerate things within a sentence.

Most uses of parentheses within citations fall into the first category. When we cite a book, we put the publication data (place, publisher, and year) in parentheses because those details are supplemental to the basic identification of the author and the book. When we cite a journal, we put the date of publication in parentheses because that is supplemental to the basic ID (author, article title, journal, issue). Following the same pattern, when we cite a website, EE suggests putting the URL (place of publication) and date of access in parentheses.

When we cite an original record and we place the date or years in parentheses, we are also signaling that the additional detail is helpful to our understanding but not necessary to the identification or location of the record.

Beyond this basic function, in both narrative writing and citation, punctuation can also vary in ways that help us grasp "units" of information. When citing records from the U.S. state and national archives, for example, we often have to deal with long strings of identifying data—file name, collection name, series name, record group name, &c &c &c, with dates being a part of any or all of these elements. In those strings, the individual units of data frequently have internal punctuation. When all are strung together, a reader might have difficulty deciding whether one unit of data is a descriptor for the preceding information or whether it is intended to be a separate element of equal or greater hierarchy. In these cases, parentheses can be used to inform a reader that certain words or dates modify or define what comes before them.

In other instances, parentheses are used in modern citations because that usage was instituted generations or centuries earlier by pioneers in a particular field and the practice has become entrenched. For example, when legal case reporters are cited Bluebook style, the year of the court decision is placed in parentheses, even though that year is an essential detail. Ditto for the year of statutes. On the other hand, when we cite international treaties a la Bluebook we do not use parentheses around the equally essential date of the treaty; but when we cite U.N. resolutions, the parentheses are expected. (Bluebook 1.4, Rules 10, 12, 21.4 and 21.7). 

Then, of course, other considerations fall into the category of "niceties," like avoiding the consecutive placement of two separate sets of parenthetical data—as when authors create book titles that place the last few words in parentheses and thereby cause such structural infelicities as Mike Muddler, Moonshining in Massachusetts (1750–1900)(Middleborough: Muckraker Press, 1905), 123. 

And all of this, of course, is why eyes roll when most people hear the words punctuation and citation used in the same breath.

The bottom line, rraymond, would have to be this: As a brilliant software engineer, you value hard and fast rules (and for good reason); but both humans and their records have their vagaries. EE eliminates many of these inconsistencies within citations to various types of materials; but "rules" are like much else in life: when one clashes with another, one or the other will likely have to yield.