writing - Why, in old books, are dates often given with the years redacted?

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:

Letter 1

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(mostly older) books, and my only hypothesis is that it is/was a fashionable attempt to try not to make the book seem outdated quite so quickly; or as a sort of faux attempt to feign respect for privacy, within the world of the novel itself.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...).