How to use two different date formats: yymmdd and dd/mm/yyyy?

Question

I would like to be able to use, in the same document, these two date formats:

• dd/mm/yyyy (as default format)
• yymmdd (as "local" format)

Why I want that?

Because I would like to use the "local" format to define the document reference code (e.g. mydocumenttitle_yymmdd.pdf) but in "normal" text, I would like to use the default dd/mm/yyyy format with the command \today.

First attempt

For the moment I wasn't able to do better than this:

\documentclass{article}
\usepackage{datetime}
\newdate{specialdate}{\day}{\month}{\year}
\usepackage[datesep={}]{datetime2} 

\begin{document}
Format with dd/mm/yyyy: {\ddmmyyyydate\displaydate{specialdate}}

Format with yymmdd: \today
\end{document}

Which gives the following result:

But there are the following problems:

1. The yymmdd format is not correct because I achieve only yyyymmdd. How to obtain the date format with only two digits for the year: yymmdd?

2. The use of command \today doens't give the default format but the yymmdd one. Is there a way to define \today with the default format (i.e. dd/mm/yyyy) and another command for the "local" format (i.e. yymmdd)?

3. It use the two packages datetime and datetime2. Is it possible to use only one package?

Answer 1

Try this code.

The package datetime is obsolete. Only datetime2 is needed.

Two styles have been defined to present the desired date formats --default (ddmmyyyyx) and local (yymmdd)--.

Using \DTMsetstyle{<name>} in the preamble sets the default style.

\documentclass{article}

\usepackage{datetime2}  

    
\DTMnewstyle{yymmdd}{%
\renewcommand*\DTMdisplaydate[4]{%
\DTMtwodigits{##1}\DTMtwodigits{##2}\DTMtwodigits{##3}% 
}
\renewcommand*{\DTMDisplaydate}{\DTMdisplaydate}%
}
{}{}{}

\newcommand{\daymonthyearsep}{/}    % define the separator
\DTMnewstyle{ddmmyyyyx}{%
\renewcommand*\DTMdisplaydate[4]{%
\DTMtwodigits{##3}\daymonthyearsep\DTMtwodigits{##2}\daymonthyearsep\number##1%
}%
\renewcommand*{\DTMDisplaydate}{\DTMdisplaydate}%
}
{}{}{}  


\DTMsetstyle{ddmmyyyyx}% set as default style

\begin{document}

Default style \emph{ddmmyyyyx}: \today
\bigskip

Now a local style \emph{yymmdd}: {\DTMsetstyle{yymmdd} \today}%local style <<<<<<<<<<<<<<<<
\bigskip

Default style again \emph{ddmmyyyyx}:   \today
    
\end{document}