Anti-chronological bibliography with sorting=ydnt and usage of sortyear

Question

I'm trying to get a anti-chronological bibliography, that means I just want to appear the newest items at the top. After reading the the answer for this question I assumed, that the usage the sorting scheme ydnt and of sortyear (in the format 2012-1, 2012-2 and so on, where 2012-1 is older than 2012-2) should do that.

But it does not, it seems that only the first four digits of sortyear or the entry in year are used (which are in fact the same).

So, why that? And how could I get a new-to-old sorted bibliography without declaring a new sorting scheme? (Doing that I get the desired result.)

\documentclass{article}
\usepackage{filecontents}
\usepackage[english]{babel}
\usepackage[backend=biber,sorting=ydnt]{biblatex}

\begin{filecontents}{refs.bib}
@MISC{2012_1,
  author = {Me},
  title = {A Oldest, should be below},
  year = {2012},
  sortyear = {2012-1},
}
@MISC{2012_2,
  author = {Me},
  title = {C Second oldest, should be in middle},
  year = {2012},
  sortyear = {2012-2},
}

@MISC{2012_3,
  author = {Me},
  title = {B Newest, should be above},
  year = {2012},
  sortyear = {2012-3},
}
}\end{filecontents}

\addbibresource{refs.bib}

\begin{document}
\nocite{*}
\printbibliography
\end{document}

Answer 1

Firstly, note that "2012-1" is not a valid ISO 8601 date format which biblatex expects in the date field. It's also not a valid year field for obvious reasons. This is why we still have the month field. So, to make this work, it's fairly easy. Change the format of your .bib entries to e.g:

@MISC{2012_1,
  author = {Me},
  title = {A Oldest, should be below},
  year = {2012},
  month = {1}
}
@MISC{2012_2,
  author = {Me},
  title = {C Second oldest, should be in middle},
  year = {2012},
  month = {2}
}

@MISC{2012_3,
  author = {Me},
  title = {B Newest, should be above},
  year = {2012},
  month = {3}
}

and then define a new "year month descending" sorting spec in your preamble. The name is arbitrary but call it say "ymdnt":

\DeclareSortingScheme{ymdnt}{
  \sort{
    \field{presort}
  }
  \sort[final]{
    \field{sortkey}
  }
  \sort[direction=descending]{
    \field[strside=left,strwidth=4]{sortyear}
    \field[strside=left,strwidth=4]{year}
    \literal{9999}
  }
  \sort[direction=descending]{
    \field{month}
    \literal{9999}
  }
  \sort{
    \field{sortname}
    \field{author}
    \field{editor}
    \field{translator}
    \field{sorttitle}
    \field{title}
  }
  \sort{
    \field{sorttitle}
    \field{title}
  }
}

This is just a copy of the "ydnt" scheme from biblatex.def with a sort on the month field added after year. Then load biblatex with sorting=ymdnt and it all works nicely.