Date format (in bib entry) / bibelatex biber

Question

I'm using biber and biblatex and I wanna print all thesis prior to 2013. When I use

year = {2010}

entry in the .bib file, every think works fine. However, if I use

date = {2010-11-25}

instead of year, my entry is ignored. I'm respecting the date format, right? So what is wrong? Here is a MWE.

\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} 

} %  date = {2010-11-25}
\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]
}}}

\begin{document}
Hello world.
    \nocite{*}
    \printbibliography[type=thesis,keyword=before2013,title=Thesis,resetnumbers=true]
\end{document}

Answer 1

Instead of Biber's sourcemapping you can compare the year field with biblatex in the document. (We can use year here regardless of whether you used year or date since a date field is always split up into its components when it is written to the .bbl.)

We define a bibcheck

\defbibcheck{before2013}{%
  \iffieldint{year}
    {\ifnumless{\thefield{year}}{2013}
       {}
       {\skipentry}}
    {\skipentry}}

and use that check instead of the keyword

\printbibliography[type=thesis,check=before2013,title=Thesis,resetnumbers=true]

Answer 2

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.