Expanding macro arguments in newcommand

Question

I'd like to use datetime's \monthname as part of a command; consider this snippet of command line pdflatex use:

$ pdflatex
This is pdfTeX, Version 3.1415926-1.40.11 (TeX Live 2010)
 restricted \write18 enabled.
**\documentclass{article}
entering extended mode
LaTeX2e <2009/09/24>
Babel <v3.8l> and hyphenation patterns for english, dumylang, nohyphenation, lo
aded.

*\usepackage{datetime}
(/PATH/TO/texlive/texmf-dist/tex/latex/base/article.cls
Document Class: article 2007/10/19 v1.4h Standard LaTeX document class
(/PATH/TO/texlive/texmf-dist/tex/latex/base/size10.clo))

*\newdate{someDate}{30}{05}{2011}
(/PATH/TO/texlive/texmf-dist/tex/latex/datetime/datetime.sty
(/PATH/TO/texlive/texmf-dist/tex/latex/fmtcount/fmtcount.sty
(/PATH/TO/texlive/texmf-dist/tex/latex/base/ifthen.sty)
(/PATH/TO/texlive/texmf-dist/tex/latex/graphics/keyval.sty)
(/PATH/TO/texlive/texmf-dist/tex/latex/amsmath/amsgen.sty)
(/PATH/TO/texlive/texmf-dist/tex/latex/fmtcount/fc-english.def)
(/PATH/TO/texlive/texmf-dist/tex/latex/fmtcount/fc-USenglish.def)
No configuration file fmtcount.cfg found.
))

*\newcommand\someDateStr{\getdateday{someDate} \monthname[\getdatemonth{someDate}] \getdateyear{someDate}}

*\typeout{\someDateStr}
30 \monthname[05] 2011

*\typeout{\monthname[\getdatemonth{someDate}]}
\monthname[05]

So, it looks like in the construct \monthname[\getdatemonth{someDate}], Latex will expand only \getdatemonth{someDate}, but not the \monthname part; I'm suspecting it has something to do with the square brackets as arguments, because if I try to use edef instead, I get an error:

*\edef\someDateStr{\getdateday{someDate} \monthname[\getdatemonth{someDate}] \getdateyear{someDate}}
! Argument of \reserved@a has an extra }.
<inserted text> 
                \par 
<*> ...atemonth{someDate}] \getdateyear{someDate}}

? 

So, the question is: how can I use \monthname[\getdatemonth{someDate}] as a part of a newcommand, such that it results with (in this case) "May"?

Thanks in advance for any responses,
Cheers!

Answer 1

The main problem with datetime is that it is oriented towards printing dates rather than manipulating them.

A "completely expandable" version would be

\makeatletter
\newcommand{\xnewdate}[4]{\@namedef{date@#1}{/#2/#3/#4/}}
\def\xgetdateday#1{\expandafter\expandafter\expandafter\xget@I\csname date@#1\endcsname}
\def\xgetdatemonth#1{\expandafter\expandafter\expandafter\xget@II\csname date@#1\endcsname}
\def\xgetdateyear#1{\expandafter\expandafter\expandafter\xget@III\csname date@#1\endcsname}
\def\xget@I/#1/#2/#3/{#1}
\def\xget@II/#1/#2/#3/{#2}
\def\xget@III/#1/#2/#3/{#3}
\def\xgetdatemonthname#1{%
\ifcase\number\xgetdatemonth{#1}\relax
\or January%
\or February%
\or March%
\or April%
\or May%
\or June%
\or July%
\or August%
\or September%
\or October%
\or November%
\or December%
\fi}
\makeatother

\typeout{\xgetdateday{somedate} \xgetdatemonthname{somedate} \xgetdateyear{somedate}}