silly question, and I'm not sure this is even necessarily the right forum, but it's the most appropriate on StackExchange, so here we are.
Why is it, in older books, that years are sometimes redacted and replaced with a dash when writing the date in letters and so forth?
Here is an example, from Mary Shelley's Frankenstein:
St. Petersburgh, Dec. 11th, 17--
TO Mrs. Saville, England
You will rejoice to hear that no disaster has accompanied...
I've seen this in many (mostly older) books, and my only hypothesis is that it is/was a fashionable attempt to try not to make the book seem outdated quite so quickly; or as a sort of faux attempt to feign respect for privacy, within the world of the novel itself.
In a similar vein, in Frankenstein, several curse words (D--n) are also redacted. I assume this is a sort of Victorian modesty in not printing profanity, but if I'm wrong, I'd love to be corrected on that, as well.
EDIT: I just received this back from the reference librarian (libraries are so great!):
It seems that there is no definitive explanation, but several explanations seem to come up over and over again. I am including the best of what I found online, rather than some of the random information that is posted (though, I will include one online discussion that might be interesting for you all the same).
From author John Barth: http://www.colby.edu/~isadoff/ss/barth.doc "Initials, blanks, or both were often substituted for proper names in nineteenth century fiction to enhance the illusion of reality. It is as if the author felt it necessary to delete the names for reasons of tact or legal liability. Interestingly, as with other aspects of realism, it is an illusion that is being enhanced, by purely artificial means."
Electronic Labyrinth: Postmodernism and the Postmodern Novel http://elab.eserver.org/hfl0256.html "... a literary convention of the time when many books and pamphlets were written criticising the government of the day, or important figures, by using false names... Some rather scurrilous stories were also printed which were thinly veiled parodies or criticisms of important figures. So when Jane Austen wrote the __shire regiment, or the Earl of__, she was a)avoiding the pitfall of being accused of inaccuracy and b) avoiding the pitfall of being accused of criticism of some important political figures."
Here is that discussion I mentioned: Republic of Pemberley Archive: More or less: http://www.pemberley.com/bin/archives/regarc1.pl?read=9221
Here is one more online discussion with a very nice and referenced answer, though the source page is no longer available. It discusses the use of this convention in epistolary novels (novels written in the form of letters): http://answerpool.com/eve/forums/a/tpc/f/436601891/m/6931055141
Since I think a couple of these links came up in the answers below, I'm just going to upvote them all and mark as answered the closest one (not that it was a quiz; but there were many good suggestions, and I can only mark one as the answer...).