The ultimate solution is this:
Search “spec” files in your Ruby directory:
*C:\Ruby187\lib\ruby\gems\1.8\specifications*
If a gemspec file contains something like this:
s.date = %q{2011-10-13 00:00:00.000000000Z}
Then delete the 00:00:00.000000000Z part:
s.date = %q{2011-10-13}
After saving those gemspec files, the problem is solved.