What is the correct way to handle approximate dates in biblatex?

Question

I need to cite something for which only an approximate date is known i.e. 'circa YYYY'. Normally, I would write this as 'cYYYY'. However, if I try putting this in the date field of my .bib entry, (I just discovered rather too late that...) I get no date at all. What is the correct way to handle such cases?

If I use the year field rather than date, the output is correct, but I'm not sure whether this is the right way to handle such cases given that year etc. are provided primarily for backwards compatibility.

\documentclass{article}
\usepackage{biblatex}
\begin{filecontents}{\jobname.bib}
  @book{mybook,
    author      =   {Author, A. N.},
    date        =   {c1550},
    title       =   {Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
\end{filecontents}
\bibliography{\jobname}
\begin{document}
  \fullcite{mybook}
\end{document}

EDIT

Setting aside the question of whether the use of year is deprecated or intended for this kind of case, one reason that simply using year is not a satisfactory solution is precisely because BibLaTeX does not parse it very intelligently. Consider the following example:

\documentclass{article}
\usepackage[style=authoryear]{biblatex}
\begin{filecontents}{\jobname.bib}
  @book{book,
    author      =   {Author, A. N.},
    year        =   {c1550},
    title       =   {Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{prequel,
    author      =   {Author, A. N.},
    date        =   {1540},
    title       =   {Early Tudor Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{sequel,
    author      =   {Author, A. N.},
    year        =   {c1560},
    title       =   {Earlier Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{afterthought,
    author      =   {Author, A. N.},
    date        =   {1570},
    title       =   {Greatest Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
\end{filecontents}
\bibliography{\jobname}
\begin{document}
  \nocite{*}
  \printbibliography
\end{document}

The entries here should be sorted in date order, but they are not:

Answer 1

In my opinion year is the right field.

Apart from my opinion, if you want your "circa " years to be computed as normal ones by the sorting processor, add the following lines in your preamble:

\DeclareSourcemap{
  \maps[datatype=bibtex]{
      \map[overwrite=false]{
          \step[fieldsource=year]
          \step[fieldset=sortyear, origfieldval, final]
          \step[fieldsource=sortyear, match={c}, replace={}]
    }
  }
}

This lets the processor think that the c preceeding the year is not there...

MWE (used \addbibresource{\jobname.bib} instead of \bibliography{\jobname})

\documentclass{article}
\usepackage[style=authoryear]{biblatex}
\begin{filecontents}{\jobname.bib}
  @book{book,
    author      =   {Author, A. N.},
    year        =   {c1550},
    title       =   {Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{prequel,
    author      =   {Author, A. N.},
    date        =   {1540},
    title       =   {Early Tudor Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{sequel,
    author      =   {Author, A. N.},
    year        =   {c1560},
    title       =   {Earlier Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{afterthought,
    author      =   {Author, A. N.},
    date        =   {1570},
    title       =   {Greatest Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
\end{filecontents}

\DeclareSourcemap{
  \maps[datatype=bibtex]{
      \map[overwrite=false]{
          \step[fieldsource=year]
          \step[fieldset=sortyear, origfieldval, final]
          \step[fieldsource=sortyear, match={c}, replace={}]
    }
  }
}


\addbibresource{\jobname.bib}

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

Output

Answer 2

It's now possible to handle approximate dates with biblatex 3.5. According to the documentation (§2.3.8, p. 36) and the example file 96-dates.tex, you can use the date field writing date = {1550~}

\documentclass{article}
\usepackage[style=authoryear, 
    datecirca=true,
    ]{biblatex}
\begin{filecontents}{\jobname.bib}
  @book{book,
    author      =   {Author, A. N.},
    date        =   {1550~},
    title       =   {Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{prequel,
    author      =   {Author, A. N.},
    date        =   {1540},
    title       =   {Early Tudor Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{sequel,
    author      =   {Author, A. N.},
    date        =   {1560~},
    title       =   {Earlier Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
  @book{afterthought,
    author      =   {Author, A. N.},
    date        =   {1570},
    title       =   {Greatest Elizabethan Escapes},
    publisher   =   {Quills \& Offspring},
    location    =   {Hamlet, British Isles}}
\end{filecontents}
\bibliography{\jobname}
\begin{document}
  \nocite{*}
  \printbibliography
\end{document}

Whit the following result:

Edit for styling customization:

Adding the two following lines in the preamble (see documentation §3.10.1, pp. 108-109):

\renewcommand{\datecircadelim}{}
\DefineBibliographyStrings{english}{circa = c}