Your \maps
only sets keyword=before2013
if year
is 1997 up to 2012 but not if date
is 1997-01-01 up to 2012-12-31. The index is only printed if keyword=before2013
. So the entry would not be printed, if date
instead of year
is used.
You can add a similar map for date
like you have for year
:
\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@THESIS{a,
AUTHOR = {Myself},
TITLE = {Me, myself and I},
Institution = {My University},
type = {Ph.D. thesis},
year = {2010}
}
@THESIS{b,
AUTHOR = {Another},
TITLE = {Another, one and nobody},
Institution = {My University},
type = {Ph.D. thesis},
date = {2010-11-25}
}
@THESIS{c,
AUTHOR= {Never},
TITLE = {I'm to late},
Institution = {My University},
type = {Ph.D. thesis},
date = {2017-01-01}
}
\end{filecontents*}
\usepackage[backend=biber,defernumbers=true,sorting=ydnt]{biblatex}
\addbibresource{\jobname.bib}
\DeclareSourcemap{
\maps[datatype=bibtex]{
\map[overwrite=true]{
\step[fieldsource=year, match=\regexp{199([7-9])|200([0-9])|201([0-2])},final]
\step[fieldset=keywords, fieldvalue={,}, append]
\step[fieldset=keywords, fieldvalue={before2013}, append]
}
\map[overwrite=true]{
\step[fieldsource=date, match=\regexp{199([7-9])|200([0-9])|201([0-2])},final]
\step[fieldset=keywords, fieldvalue={,}, append]
\step[fieldset=keywords, fieldvalue={before2013}, append]
}
}
}
\begin{document}
Hello world.
\nocite{*}
\printbibliography[type=thesis,keyword=before2013,title=Thesis,resetnumbers=true]
\end{document}

Another solution would be to use a check, because checks are done, when the date already has been splitted:
\documentclass{article}
\usepackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@THESIS{a,
AUTHOR = {Myself},
TITLE = {Me, myself and I},
Institution = {My University},
type = {Ph.D. thesis},
year = {2010}
}
@THESIS{b,
AUTHOR = {Another},
TITLE = {Another, one and nobody},
Institution = {My University},
type = {Ph.D. thesis},
date = {2010-11-25}
}
@THESIS{c,
AUTHOR= {Never},
TITLE = {I'm to late},
Institution = {My University},
type = {Ph.D. thesis},
date = {2017-01-01}
}
\end{filecontents*}
\usepackage[backend=biber,defernumbers=true,sorting=ydnt]{biblatex}
\addbibresource{\jobname.bib}
\defbibcheck{before2013}{%
\iffieldint{year}% If an interger field year exists
{
\ifnumless{\thefield{year}}{2013}% and the field value is less than 2013
{}% do nothing
{\skipentry}% otherwise skip the entry
}
{\skipentry}% skip the entrie also if there isn't an integer field year
}
\begin{document}
Hello world.
\nocite{*}
\printbibliography[type=thesis,check=before2013,title=Thesis,resetnumbers=true]
\end{document}
The result is the same.