Insert relative date and time specifications?

Question

How can I specify a relative date and let LaTeX insert the resulting absolute one?

For example like this:

\somedatecommand{next thursday}

Which should result in:

Thursday, 20.10.2011

Or:

20.10.2011

Great would be also babel support (e.g. for non-english documents).

Answer 1

You could use the datetime package to format the dates and quoting form the package documentation:

As from version 2.42, the datetime package is now compatible with babel, however you must load the datetime package after the babel package.

The harder part is determining the date to print. This can be achieved using \AdvanceDate[n] macro from the advdate package to advance the date by a specified number of days. Then it is just a matter of determining how many days we need to advance to get to "next day-of-week".

To start from a day other than the current day one can use \SetDate[dd/mm/yyyy].

\documentclass{article}
\usepackage{xstring}% Needed for \IfStrEqCase
\usepackage{datenumber}% Date formatting
\usepackage{advdate}% Needed for \AdvanceDate
\usepackage{pgf}% For math
\usepackage[ddmmyyyy]{datetime}
\newdateformat{mydate}{\twodigit{\THEDAY}.\twodigit{\THEMONTH}.\THEYEAR}

\newcounter{dateOffset}%
\newcommand*{\SetDateOffsetForNext}[1]{%
\pgfmathsetcounter{dateOffset}{7-int(\the\value{datedayname})}% Initialize
\IfStrEqCase{#1}{%
    {mon}{\pgfmathsetcounter{dateOffset}{int(mod(\the\value{dateOffset}+1,7))}}%
    {tue}{\pgfmathsetcounter{dateOffset}{int(mod(\the\value{dateOffset}+2,7))}}%
    {wed}{\pgfmathsetcounter{dateOffset}{int(mod(\the\value{dateOffset}+3,7))}}%
    {thu}{\pgfmathsetcounter{dateOffset}{int(mod(\the\value{dateOffset}+4,7))}}%
    {fri}{\pgfmathsetcounter{dateOffset}{int(mod(\the\value{dateOffset}+5,7))}}%
    {sat}{\pgfmathsetcounter{dateOffset}{int(mod(\the\value{dateOffset}+6,7))}}%
    {sun}{\pgfmathsetcounter{dateOffset}{int(mod(\the\value{dateOffset}+7,7))}}%
    }[\PackageError{\SetDateOffsetForNext}{Do not know "#1" as day of week}{}]%
    \IfEq{\the\value{dateOffset}}{0}{\pgfmathsetcounter{dateOffset}{7}}{}%
}%

\newcommand*{\PrintDateForNext}[1]{%
    \SetDateOffsetForNext{#1}% Determine date offset
    \AdvanceDate[\value{dateOffset}]% Advance to specified day
    \mydate\today% Print specified date
    \AdvanceDate[-\value{dateOffset}]% Restore current day
    is next #1\par% debug output
}

\begin{document}
\noindent
Working from current day:
Today is \mydate\today\par
\PrintDateForNext{mon}\par
\PrintDateForNext{tue}\par
\PrintDateForNext{wed}\par
\PrintDateForNext{thu}\par
\PrintDateForNext{fri}\par
\PrintDateForNext{sat}\par
\PrintDateForNext{sun}\par

\bigskip\noindent
Back to ``May the Fourth Be With You":
\SetDate[04/05/1977]% dd/mm/yyyy
Today is \mydate\today\par
\PrintDateForNext{mon}\par
\PrintDateForNext{tue}\par
\PrintDateForNext{wed}\par
\PrintDateForNext{thu}\par
\PrintDateForNext{fri}\par
\PrintDateForNext{sat}\par
\PrintDateForNext{sun}\par
\end{document}