How to insert the next month, like /today?

Question

I'd like to create a command that would insert the next month (in relation to the current one), like so:

\command -> November

analogously to \today.

• The command cannot be hardcoded, as the document switches between languages (the same way it works with \today).
• I'd like to use this command within the preamble.
• The output should be localized, i.e, elsewhere, \today should behave as usual.

What's the best way to do this?

Answer 1

Got something like this to work:

\usepackage{datetime}
\usepackage{advdate}

\newdateformat{mydate}{\monthname[\THEMONTH]}

(...)

{\AdvMonth{1} \mydate \today}

Answer 2

Here's a solution using datetime2 and a little modular arithmetic.

\documentclass{article}
\usepackage[french,english]{babel}
\usepackage[T1]{fontenc}
\usepackage[calc]{datetime2}
\ExplSyntaxOn
\NewDocumentCommand{\nextmonth} {} {
\int_eval:n {12 - \int_mod:nn {\month}{12}}{=}{0}}
\DTMmonthname{\nxtmnth:n}}
\ExplSyntaxOff
\begin{document}
Today is: \today

Next month is \nextmonth
\selectlanguage{french}

En français: \nextmonth

\end{document}

Answer 3

For doing algebra modulo twelve you can use expl3's \int_mod:nn where the result's absolute value is smaller than the absolute value of the module and where the sign of the result is the same as with the expression whose integer remainder shall be calculated:

Probably providing the result of the calculation

12 - (
         (
             % With \int_mod:nn for doing the mod-12-calculation this
             % expression yields a non-negative number...
             12 - ( 
                      % ... because this is in range -11..+11 :
                      (   ⟨number of current month⟩
                        + ⟨amount of months to add or subtract⟩
                      ) mod 12 
                  )
         ) mod 12  
         % With \int_mod:nn the result of mod-12-calculation of a
         % non-negative number is in range 0..11.
     )

to datetime2's \DTMmonthname does the trick:

\documentclass{article}
\usepackage[french,english]{babel}
\usepackage[T1]{fontenc}
\usepackage[calc]{datetime2}
\ExplSyntaxOn
\NewDocumentCommand{\MonthInRelationToCurrentMonth} {m} {
  \exp_args:Nf \DTMmonthname
               {\int_eval:n {12-\int_mod:nn {12-(\int_mod:nn{\month+(#1)}{12})}{12}}}
}
\ExplSyntaxOff
\newcommand\NextMonth{\MonthInRelationToCurrentMonth{1}}%
\newcommand\PreviousMonth{\MonthInRelationToCurrentMonth{-1}}%
\newcommand\ThisMonth{\MonthInRelationToCurrentMonth{0}}%

\begin{document}
\bigskip\hrule\bigskip
\noindent In English
\bigskip\hrule\bigskip
\noindent 
Today is \today.\\
Previous month was \PreviousMonth.\\
This month is \ThisMonth.\\
Next month will be \NextMonth.

\bigskip\hrule\bigskip

\noindent 
This month is \MonthInRelationToCurrentMonth{0}.\\
This month is  \MonthInRelationToCurrentMonth{+0}.\\
One month later will be \MonthInRelationToCurrentMonth{+1}.\\
Two months later will be \MonthInRelationToCurrentMonth{2}.\\
Three months later will be \MonthInRelationToCurrentMonth{3}.\\
Four months later will be \MonthInRelationToCurrentMonth{4}.\\
Five months later will be \MonthInRelationToCurrentMonth{+5}.\\
Six months later will be \MonthInRelationToCurrentMonth{+6}.\\
Seven months later will be \MonthInRelationToCurrentMonth{7}.\\
Eight months later will be \MonthInRelationToCurrentMonth{8}.\\
Nine months later will be \MonthInRelationToCurrentMonth{9}.\\
Ten months later will be \MonthInRelationToCurrentMonth{10}.\\
Eleven months later will be \MonthInRelationToCurrentMonth{11}.\\
Twelve months later will be \MonthInRelationToCurrentMonth{12}.\\
Thirteen months later will be \MonthInRelationToCurrentMonth{13}.\\
Twenty months later will be \MonthInRelationToCurrentMonth{+20}.

\bigskip\hrule\bigskip

\noindent 
This month is \MonthInRelationToCurrentMonth{0}.\\
This month is  \MonthInRelationToCurrentMonth{-0}.\\
One month earlier was \MonthInRelationToCurrentMonth{-1}.\\
Two months earlier was \MonthInRelationToCurrentMonth{-2}.\\
Three months earlier was \MonthInRelationToCurrentMonth{-3}.\\
Four months earlier was \MonthInRelationToCurrentMonth{-4}.\\
Five months earlier was \MonthInRelationToCurrentMonth{-5}.\\
Six months earlier was \MonthInRelationToCurrentMonth{-6}.\\
Seven months earlier was \MonthInRelationToCurrentMonth{-7}.\\
Eight months earlier was \MonthInRelationToCurrentMonth{-8}.\\
Nine months earlier was \MonthInRelationToCurrentMonth{-9}.\\
Ten months earlier was \MonthInRelationToCurrentMonth{-10}.\\
Eleven months earlier was \MonthInRelationToCurrentMonth{-11}.\\
Twelve months earlier was \MonthInRelationToCurrentMonth{-12}.\\
Thirteen months earlier was \MonthInRelationToCurrentMonth{-13}.\\
Twenty months earlier was \MonthInRelationToCurrentMonth{-20}.

\newpage
\selectlanguage{french}

\bigskip\hrule\bigskip
\noindent En français:
\bigskip\hrule\bigskip
\noindent 
Aujourd'hui, c'est \today.\\
Le mois précédent était \PreviousMonth.\\
Ce mois-ci est \ThisMonth.\\
Le mois prochain sera \NextMonth.

\bigskip\hrule\bigskip

\noindent 
Ce mois-ci, c'est \MonthInRelationToCurrentMonth{0}.\\
Ce mois-ci, c'est \MonthInRelationToCurrentMonth{+0}.\\
Un mois plus tard sera \MonthInRelationToCurrentMonth{+1}.\\
Deux mois plus tard sera \MonthInRelationToCurrentMonth{2}.\\
Trois mois plus tard sera \MonthInRelationToCurrentMonth{3}.\\
Quatre mois plus tard sera \MonthInRelationToCurrentMonth{4}.\\
Cinq mois plus tard sera \MonthInRelationToCurrentMonth{+5}.\\
Six mois plus tard sera \MonthInRelationToCurrentMonth{+6}.\\
Sept mois plus tard sera \MonthInRelationToCurrentMonth{7}.\\
Huit mois plus tard sera \MonthInRelationToCurrentMonth{8}.\\
Neuf mois plus tard sera \MonthInRelationToCurrentMonth{9}.\\
Dix mois plus tard plus tard sera \MonthInRelationToCurrentMonth{10}.\\
Onze mois plus tard plus tard sera \MonthInRelationToCurrentMonth{11}.\\
Douze mois plus tard plus tard sera \MonthInRelationToCurrentMonth{12}.\\
Treize mois plus tard plus tard sera \MonthInRelationToCurrentMonth{13}.\\
Vingte mois plus tard plus tard sera \MonthInRelationToCurrentMonth{+20}.

\bigskip\hrule\bigskip

\noindent 
Ce mois-ci, c'est \MonthInRelationToCurrentMonth{0}.\\
Ce mois-ci, c'est \MonthInRelationToCurrentMonth{+0}.\\
Un mois plus tôt, c'était \MonthInRelationToCurrentMonth{-1}.\\
Deux mois plus tôt, c'était \MonthInRelationToCurrentMonth{-2}.\\
Trois mois plus tôt, c'était \MonthInRelationToCurrentMonth{-3}.\\
Quatre mois plus tôt, c'était \MonthInRelationToCurrentMonth{-4}.\\
Cinq mois plus tôt, c'était \MonthInRelationToCurrentMonth{-5}.\\
Six mois plus tôt, c'était \MonthInRelationToCurrentMonth{-6}.\\
Sept mois plus tôt, c'était \MonthInRelationToCurrentMonth{-7}.\\
Huit mois plus tôt, c'était \MonthInRelationToCurrentMonth{-8}.\\
Neuf mois plus tôt, c'était \MonthInRelationToCurrentMonth{-9}.\\
Dix mois plus tôt, c'était \MonthInRelationToCurrentMonth{-10}.\\
Onze mois plus tôt, c'était \MonthInRelationToCurrentMonth{-11}.\\
Douze mois plus tôt, c'était \MonthInRelationToCurrentMonth{-12}.\\
Teize mois plus tôt, c'était \MonthInRelationToCurrentMonth{-13}.\\
Vingte mois plus tôt, c'était \MonthInRelationToCurrentMonth{-20}.

\end{document}