Biblatex and entries with no date

You can add the following hooks to your preamble:

\newcommand{\mkbibnodate}{n\adddot d\adddot}
\AtEveryCitekey{\iffieldundef{labelyear}{\restorefield{labelyear}{\mkbibnodate}}{}}
\AtEveryBibitem{\iffieldundef{labelyear}{\restorefield{year}{\mkbibnodate}}{}}

With biber, you can achieve the same result with the source map:

\DeclareSourcemap{
  \maps[datatype=bibtex]{
    \map[overwrite=false]{
      \step[fieldset=date,origfieldval,final]
      \step[fieldset=eventdate,origfieldval,final]
      \step[fieldset=origdate,origfieldval,final]
      \step[fieldset=urldate,origfieldval,final]
      \step[fieldset=eventyear,origfieldval,final]
      \step[fieldset=origyear,origfieldval,final]
      \step[fieldset=urlyear,origfieldval,final]
      \step[fieldset=year,fieldvalue={n.d.},final]
      \step[fieldset=sortyear,fieldvalue={0000}]
    }
  }
}

Both of these approaches are style-independent so they should work for authoryear and any of its variants.

The above sourcemap is based on the default labelyear specification:

\DeclareLabelyear{
  \field{year}
  \field{eventyear}
  \field{origyear}
  \field{urlyear}
}

Currently \DeclareLabelyear does not support literal strings, but something like the following would be useful:

\DeclareLabelyear{
  \field{year}
  \field{eventyear}
  \field{origyear}
  \field{urlyear}
  \literal{n.d.}
}

I've added a feature request for this at the biblatex github repository.

Please try biblatex 2.6/biber 1.6 development versions from SourceForge. This set up is now the default and there is a new localisation string "nodate" which defaults to "n.d." in all languages currently. \DeclareLabelyear is now the more general:

\DeclareLabeldate{%
  \field{date}
  \field{event}
  \field{orig}
  \field{url}
  \literal{nodate}
}

but you shouldn't need to worry about that unless you are a style author using this macro. With these versions, your example gives the following without any extra coding:

Changing the "nodate" localisation string for your language will change the "n.d." in both the references and citation.

\renewbibmacro*{date+extrayear}{%
    \iffieldundef{year}
      {\printtext[parens]{n.d.}}
      {\printtext[parens]{\printdateextra}}}%

\renewbibmacro*{cite:label}{%
  \iffieldundef{label}
    {\printtext[bibhyperref]{\printtext{n.d.}}}
    {\printtext[bibhyperref]{\printfield{label}}}}