Why does \today crash in Tufte-LaTeX using 'isodate' when passed as an argument?

Question

I'm trying to use the isodate package with Tufte-LaTeX, but get

! Illegal parameter number in definition of \reserved@a.

when I use \today as an argument to certain functions (e.g. \printdate or \allcaps):

\documentclass[]{tufte-handout}    
\usepackage[iso,american]{isodate}    
\begin{document}    
\allcaps{\today}    
\end{document}

If \date is set, I get a different error

! Argument of \reserved@a has extra }.

Using \protect\today fixes the above, but I still get the errors when I try to use the date as I need to:

\documentclass[]{tufte-handout}    
\usepackage[iso,american]{isodate}    
\date{\protect\today}
%\date{5/16/1961} % Works fine if a date is provided
\begin{document} 
\makeatletter   
\printdate{\@date}
\end{document}

Answer 1

The default definition of \@date and \date is

\def\@date{\today}
\def\date#1{\gdef\@date{#1}}

This is not changed by isodate, which however redefines \today. You can check if the user has said \date{...} with

\def\kernel@date{\today}
\ifx\@date\kernel@date
  <what to do if no \date command has been given>
\else
  <what to do if \date has been given>
\fi

Of course if one says \date{21 marzo 2012}, \printdate would have a hard time in producing the date in correct form with

\printdate{\csname @date\endcsname}

(this is the same as \makeatletter\printdate{\@date}\makeatother, but more economical).

Answer 2

Bruno's answer of using \protect\today works for the \allcaps treatment (though I'm not sure why you'd want to print an ISO-format date with that spacing).

You can't use \today as an argument to \printdate. It needs to be in the format yyyy-mm-dd, mm.dd.yyyy, or dd/mm/yyyy.

\documentclass{tufte-handout}

\usepackage[iso,american]{isodate}

\begin{document}

\allcaps{\protect\today}% <-- added \protect

\printdate{2012/03/20}
\printdate{20.03.2012}
\printdate{20/03/2012}

\end{document}

Based on the comments below, here's a more complete solution:

\documentclass{article}

\usepackage[iso,american]{isodate}

\makeatletter
% Set default date to today's date (in ISO format)
\xdef\@date{\the\year-\the\month-\the\day}

% Helper macro to avoid @s in macro name
\newcommand{\thedate}{\@date}

% Helper macro to avoid more typing
\newcommand{\printthedate}{\printdate{\@date}}
\makeatother

% Optionally change the date
%\date{2010-01-31}% must be in ISO format

\begin{document}

% Prints the date set by \date or today's date by default
\printdate{\thedate}

% Same as above
\printthedate

\end{document}

First, we redefine \@date to be today's date (using the ISO format). This means that if \date isn't specified, we'll default to today's date.

Next, we've written a \thedate helper macro. This just saves us some typing in the document. Without this macro, we'd have to write \makeatletter\printdate{\@date}\makeatother instead of \printdate{\thedate}. Additionally, we've created a \printthedate macro that is equivalent to \printdate{\thedate}. (Yes, I'm a lazy typist!)

If you want to use a date other than today's date, you must specify it using in ISO format \date{yyyy-mm-dd} (or one of the other formats supported by the \printdate macro).

Finally, the document demonstrates the helper macros.

Answer 3

I would like to precise that Tufte classes redefine some macros

First \newcommand{\thedate}{\today}

and

\renewcommand*{\date}[1]{%
  \gdef\@date{#1}%
  \begingroup%
    % TODO store contents of \thanks command
    \renewcommand{\thanks}[1]{}% swallow \thanks contents
    \protected@xdef\thedate{#1}%
  \endgroup%
} 

I think you need to take care of these modifications and you can define an iso format without ìsodate` package.