In biblatex version 2.5, bibliographic entries with the YEAR field in brackets in the .bib source were still sorted in the bibliography as if the field was without brackets. Cf. the example below:
\documentclass{article}
\usepackage{filecontents}
\usepackage[style=authoryear]{biblatex}
\begin{filecontents}{\jobname.bib}
@BOOK{lennon1972,
AUTHOR = "John Lennon",
TITLE = "Peace on earth",
YEAR = "1972",
LOCATION = "London",
PUBLISHER = "Music Press"}
@BOOK{lennon1974,
AUTHOR = "John Lennon",
TITLE = "More peace on earth",
YEAR = "[1974]",
LOCATION = "London",
PUBLISHER = "Music Press"}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\nocite{lennon1972}\nocite{lennon1974}
\printbibliography
\end{document}

This is no longer the case with biblatex version 2.7. I don't know what determines their sorting anymore, but it seems like the entries with the YEAR field in brackets is always put first in the sorted list under that author.

What can I do to get the sorting behavior of biblatex version 2.5 back?
sortyearfield:YEAR = "[1974]", SORTYEAR="1974",– Gonzalo Medina Aug 02 '13 at 22:59