floruit.tex

\documentclass{article}

\usepackage{etoolbox}
\usepackage{zref}

\makeatletter

%% lifespan macros:

\newbool{fl_called}
\newcommand\fl@lifespan{}
\zref@newprop{lifespan}[]{\fl@lifespan}

\newcommand\fl@prefix{fl.~}
\newcommand\fl@infix{--}
\newcommand\fl@suffix{}

\newcommand\fl@handler[1]{%
	\ifnumcomp{#1}{<}{\fl@from}{\renewcommand\fl@from{#1}}{}%
	\ifnumcomp{#1}{>}{\fl@to}{\renewcommand\fl@to{#1}}{}%
}

\newcommand\floruitsave[1]{%
	\ifbool{fl_called}{%
		\newcommand\fl@from{10000}%
		\newcommand\fl@to{0}%
		\forlistloop{\fl@handler}{\fl@years}%
		\def\@fl{%
			\fl@prefix%
			\ifnumcomp{\fl@from}{=}{\fl@to}{\fl@to}{\fl@from\fl@infix\fl@to}%
			\fl@suffix%
		}%
		\renewcommand\fl@lifespan{\@fl}%
	}{}%
	\zref@labelbyprops{#1}{lifespan}%
}

\newcommand\floruitoutput[1]{%
	\zref@extract{#1}{lifespan}%
}

\newcommand\floruit[1]{#1\booltrue{fl_called}\listadd\fl@years{#1}}

\makeatother

%% person environment:

\newcounter{person}

\newenvironment{person}[1]{%
	\refstepcounter{person}%
	\textbf{\theperson:~#1} \textit{\floruitoutput{person:\theperson}}\par%
}{%
	\floruitsave{person:\theperson}%
}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}

\begin{person}{Old MacDonald}
Old MacDonald had a farm in \floruit{1923}, but appears to have sold it by \floruit{1940}.
\end{person}

\begin{person}{Gary Cooper}
Gary Cooper starred in High Noon (\floruit{1952})...
\end{person}

\begin{person}{John Doe}
Blah blah (\floruit{1912}), more blah (\floruit{1930}) \& finally blah (\floruit{1931}).
\end{person}

\begin{person}{Jane Doe}
No floruit here...
\end{person}



The floruit should be with the name like so:

\textbf{3: John Doe} \textit{fl. 1912--1931}\par
Blah blah (1912), more blah (1930) \& finally blah (1931).\par

\end{document}

I renamed it to lifespan as I intend to add \born{} and \died{} macros too, which if present will then override and be used instead of any \floruits.

But good point on saving from/to! It's still pretty early going for this project, so they might end up needed after all.