If you want to have a new date
-type field you do not only need to ask for the date field, but also for the corresponding datepart
fields.
You also need to define the new entry types and specify which fields are allowed for these types.
The command \printseriesdate
can be used for pretty printing our new seriesdate
.
Full MWE (adapted from your answer to Biblatex: Citing complex nested unpublished sources)
\RequirePackage{filecontents}
\begin{filecontents*}{\jobname.bib}
@archivalsource{Test,
title = {Test Source},
contains = {ITEM1 and ITEM2}}
@archivalitem{ITEM1,
title = {Transcript},
author = {J. Doe},
date = {1920-04-03},}
@archivalitem{ITEM2,
title = {Report},
author = {J. Moe},
seriesdate = {1900-01-01/1900-06-06},}
\end{filecontents*}
\begin{filecontents*}{archival.dbx}
\DeclareDatamodelEntrytypes{
archivalsource,
archivalitem}
\DeclareDatamodelFields[type=list, datatype=literal]{
contains}
\DeclareDatamodelFields[type=field, datatype=date, skipout]{
seriesdate}
\DeclareDatamodelEntryfields[archivalsource]{
title,
contains,
seriesdate,
seriesday,
seriesendday,
seriesendhour,
seriesendminute,
seriesendmonth,
seriesendseason,
seriesendsecond,
seriesendtimezone,
seriesendyear,
serieshour,
seriesminute,
seriesmonth,
seriesseason,
seriessecond,
seriestimezone,
seriesyear}
\DeclareDatamodelEntryfields[archivalitem]{
author,
title,
seriesdate,
seriesday,
seriesendday,
seriesendhour,
seriesendminute,
seriesendmonth,
seriesendseason,
seriesendsecond,
seriesendtimezone,
seriesendyear,
serieshour,
seriesminute,
seriesmonth,
seriesseason,
seriessecond,
seriestimezone,
seriesyear}
\end{filecontents*}
\documentclass[british]{article}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[datamodel=archival, backend=biber]{biblatex}
\usepackage{hyperref}
\addbibresource{\jobname.bib}
\DeclareBibliographyDriver{archivalitem}{%
\usebibmacro{bibindex}%
\usebibmacro{begentry}%
\printnames{author}%
\setunit{\addspace\textendash\space}%
\printfield{title}%
\setunit{\addspace}%
\printseriesdate
\usebibmacro{finentry}}
\DeclareListFormat{contains}{
\item \entrydata{#1}{\usedriver{}{\thefield{entrytype}}}
}
\DeclareBibliographyDriver{archivalsource}{%
\usebibmacro{bibindex}%
\usebibmacro{begentry}%
\printfield{title}%
\setunit{\addperiod\space}%
\printtext{Contains:}%
\begin{enumerate}%
\printlist{contains}%
\end{enumerate}%
\iflistundef{contains}{\finentry}{}}
\begin{document}
\nocite{*}
\printbibliography
\end{document}

date
fields need a bit more infrastructure if you want to have all the comfort thatbiblatex
offers, have a look at my answer to How can I create entirely new data types with BibLaTeX/Biber? where we also deal with date fields. – moewe Jan 17 '16 at 14:23seriesdate = {1854/1899}
should render as 1854–1899 like\printdate
does. It currently renders as just the first year. Tried editing your minimal example with some ranges for testing to no avail. – meide Jan 17 '16 at 16:14