# How to implement a \tomorrow in latex

I was asked how to implement a \tomorrow macro in TeX/LaTeX. While I could come up with a reasonably good solution, working for one document class, I don't see how that could be achieved in a document class agnostic way.

I found advdate, but even though the date arithmetic may be of use, it assumes a specific format for the date (which also is wrong for the locale I reside in).

Does anyone have a reasonably portable solution? Is this even possible?

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If your class already defines \today then the only difficult part of \tomorrow is the date arithemtic, all \tommorrow can just locally add 1 day to \year\month\day taking care of date arithemetic, and then just do \today –  David Carlisle Dec 18 '12 at 1:14
No unfortunately this is not possible as far as i see, since \today is edef'd in latex.ltx for example. otherwise i would have done \def\tomorrow{\advance\day 1\relax\today\advance -1\relax} –  Max Dec 18 '12 at 1:24
Yes latex.ltx edefs a uS english version but any language package will change that, babel's german fir example has \def\dategerman{\def\today{\number\day.~\month@german \space\number\year}} \def\dateaustrian{\def\today{\number\day.~\ifnum1=\month J\"anner\else \month@german\fi \space\number\year}} so if your preamble has \dateaustian \today will give a localised today and {add a day to \day\month\year \today} will give a localised tomorrow, you can't simply \advance\day 1 as you need to take account of month arithmetic, but perhaps that's what you meant? –  David Carlisle Dec 18 '12 at 1:55
Well yes, of course that would exclude all the date arithmetic that was fabulously solved by the other two answers (and advdate). I should have used pseudocode. I'd say adding the requirement of having to use babel is ok. –  Max Dec 18 '12 at 2:15
put your system clock forward by 1 day prior to compiling, then put it back... ;-( Up-vote me if you think this is the worst hack of all time. –  Nicholas Hamilton Dec 18 '12 at 9:07

Here is a prototype in LaTeX3; the data about tomorrow is available in the integer variables

\l_tomorrow_day_int
\l_tomorrow_month_int
\l_tomorrow_year_int


The code follows; the final macro is just an example of how the data can be used, possibly in connection with datetime.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\prop_new:N \g_tomorrow_months_prop
\bool_new:N \l_tomorrow_leap_bool
\int_new:N \l_tomorrow_day_int
\int_new:N \l_tomorrow_month_int
\int_new:N \l_tomorrow_year_int
\prop_gput:Nnn \g_tomorrow_months_prop {  1 } { 31 }
\prop_gput:Nnn \g_tomorrow_months_prop {  2 } { \bool_if:NTF \l_tomorrow_leap_bool { 29 } { 28 } }
\prop_gput:Nnn \g_tomorrow_months_prop {  3 } { 31 }
\prop_gput:Nnn \g_tomorrow_months_prop {  4 } { 30 }
\prop_gput:Nnn \g_tomorrow_months_prop {  5 } { 31 }
\prop_gput:Nnn \g_tomorrow_months_prop {  6 } { 30 }
\prop_gput:Nnn \g_tomorrow_months_prop {  7 } { 31 }
\prop_gput:Nnn \g_tomorrow_months_prop {  8 } { 31 }
\prop_gput:Nnn \g_tomorrow_months_prop {  9 } { 30 }
\prop_gput:Nnn \g_tomorrow_months_prop { 10 } { 31 }
\prop_gput:Nnn \g_tomorrow_months_prop { 11 } { 30 }
\prop_gput:Nnn \g_tomorrow_months_prop { 12 } { 31 }

\cs_new_protected:Npn \tomorrow_check_leap:n #1
{
\int_compare:nTF { 0 = \int_mod:nn { #1 } { 4 } }
{% possibly a leap year
\int_compare:nTF { 0 = \int_mod:nn { #1 } { 100 } }
{% possibly not a leap year
\int_compare:nTF { 0 = \int_mod:nn { #1/100 } { 4 } }
{% leap year
\bool_set_true:N \l_tomorrow_leap_bool
}
{% not leap year
\bool_set_false:N \l_tomorrow_leap_bool
}
}
{% leap year
\bool_set_true:N \l_tomorrow_leap_bool
}
}
{% not leap year
\bool_set_false:N \l_tomorrow_leap_bool
}
}

\cs_new_protected:Npn \tomorrow_set_tomorrow:nnn #1 #2 #3
{
\int_compare:nT { #2 = 2 } { \tomorrow_check_leap:n { #3 } }
\int_set:Nn \l_tomorrow_day_int { #1 }
\int_set:Nn \l_tomorrow_month_int { #2 }
\int_set:Nn \l_tomorrow_year_int { #3 }
\__tomorrow_incr_day:
}
\cs_new_protected:Npn \__tomorrow_incr_day:
{
\int_incr:N \l_tomorrow_day_int
\int_compare:nT
{ \l_tomorrow_day_int > \prop_get:NV \g_tomorrow_months_prop \l_tomorrow_month_int }
{
\int_set:Nn \l_tomorrow_day_int { 1 }
\__tomorrow_incr_month:
}
}
\cs_new_protected:Npn \__tomorrow_incr_month:
{
\int_incr:N \l_tomorrow_month_int
\int_compare:nT { \l_tomorrow_month_int > 12 }
{
\int_set:Nn \l_tomorrow_month_int { 1 }
\int_incr:N \l_tomorrow_year_int
}
}
\cs_generate_variant:Nn \prop_get:Nn { NV }

\NewDocumentCommand{\printtomorrowof}{mmm}
{
\tomorrow_set_tomorrow:nnn { #1 } { #2 } { #3 }
Today~it~is~
\int_to_arabic:n { #3 }/
\int_to_arabic:n { #2 }/
\int_to_arabic:n { #1 },~
tomorrow~it~is~
\int_to_arabic:n { \l_tomorrow_year_int }/
\int_to_arabic:n { \l_tomorrow_month_int }/
\int_to_arabic:n { \l_tomorrow_day_int }
\par
}
\ExplSyntaxOff

\begin{document}

\printtomorrowof{\day}{\month}{\year}
\printtomorrowof{30}{10}{2012}
\printtomorrowof{31}{10}{2012}
\printtomorrowof{31}{12}{2012}
\printtomorrowof{28}{2}{2012}
\printtomorrowof{28}{2}{2013}
\printtomorrowof{28}{2}{1900}
\printtomorrowof{28}{2}{2000}

\end{document}


As you see, leap years are correctly recognized. Only Gregorian calendar, of course.

In order to define a suitable \tomorrow command, you can add (before \ExplSyntaxOn) a babel version

\NewDocumentCommand{\tomorrow}{}
{
\tomorrow_set_tomorrow:nnn { \day } { \month } { \year }
\group_begin:
\day = \l_tomorrow_day_int
\month = \l_tomorrow_month_int
\year = \l_tomorrow_year_int
\today
\group_end:
}


or a datetime version (requires package datetime, of course)

\NewDocumentCommand{\tomorrow}{}
{
\tomorrow_set_tomorrow:nnn { \day } { \month } { \year }
\formatdate { \l_tomorrow_day_int }
{ \l_tomorrow_month_int }
{ \l_tomorrow_year_int }
}


This is, of course, overkill if one wants only tomorrow's date. The macros actually allow to compute any date from a given one, given the interval (positive or negative). One might make expandable also the "reverse" from a Julian date to the form "Day/Month/Year", but it would be very slow.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\juliandate}{ m m m }
{
\juliandate_calc:nnnn { #1 } { #2 } { #3 } { \use:n }
}
\NewDocumentCommand{\storejuliandate}{ s m m m m }
{
\IfBooleanTF{#1}
{
\juliandate_calc:nnnn { #3 } { #4 } { #5 } { \cs_set:Npx #2 }
}
{
\juliandate_calc:nnnn { #3 } { #4 } { #5 } { \cs_new:Npx #2 }
}
}
\cs_new:Npn \juliandate_calc:nnnn #1 #2 #3 #4 % #1 = day, #2 = month, #3 = year, #4 = what to do
{
#4
{
\int_eval:n
{
#1 +
\int_div_truncate:nn { 153 * (#2 + 12 * \int_div_truncate:nn { 14 - #2 } { 12 } - 3) + 2 } { 5 } +
365 * (#3 + 4800 - \int_div_truncate:nn { 14 - #2 } { 12 } ) +
\int_div_truncate:nn { #3 + 4800 - \int_div_truncate:nn { 14 - #2 } { 12 } } { 4 } -
\int_div_truncate:nn { #3 + 4800 - \int_div_truncate:nn { 14 - #2 } { 12 } } { 100 } +
\int_div_truncate:nn { #3 + 4800 - \int_div_truncate:nn { 14 - #2 } { 12 } } { 400 } -
32045
}
}
}

\tl_new:N \l__juliandate_g_tl
\tl_new:N \l__juliandate_dg_tl
\tl_new:N \l__juliandate_c_tl
\tl_new:N \l__juliandate_dc_tl
\tl_new:N \l__juliandate_b_tl
\tl_new:N \l__juliandate_db_tl
\tl_new:N \l__juliandate_a_tl
\tl_new:N \l__juliandate_da_tl
\tl_new:N \l__juliandate_y_tl
\tl_new:N \l__juliandate_m_tl
\tl_new:N \l__juliandate_d_tl
\int_new:N \l_juliandate_day_int
\int_new:N \l_juliandate_month_int
\int_new:N \l_juliandate_year_int

\cs_new:Npn \__juliandate_set:nn #1 #2
{
\tl_set:cx { l__juliandate_#1_tl } { \int_eval:n { #2 } }
}
\cs_new:Npn \__juliandate_use:n #1
{
\tl_use:c { l__juliandate_#1_tl }
}
\cs_new_protected:Npn \juliandate_reverse:n #1
{
\__juliandate_set:nn { g }
{ \int_div_truncate:nn { #1 + 32044 } { 146097 } }
\__juliandate_set:nn { dg }
{ \int_mod:nn { #1 + 32044 } { 146097 } }
\__juliandate_set:nn { c }
{ \int_div_truncate:nn { ( \int_div_truncate:nn { \__juliandate_use:n { dg } } { 36524 } + 1) * 3 } { 4 } }
\__juliandate_set:nn { dc }
{ \__juliandate_use:n { dg } - \__juliandate_use:n { c } * 36524 }
\__juliandate_set:nn { b }
{ \int_div_truncate:nn { \__juliandate_use:n { dc } } { 1461 } }
\__juliandate_set:nn { db }
{ \int_mod:nn { \__juliandate_use:n { dc } } { 1461 } }
\__juliandate_set:nn { a }
{ \int_div_truncate:nn { ( \int_div_truncate:nn { \__juliandate_use:n { db } } { 365 } + 1) * 3 } { 4 } }
\__juliandate_set:nn { da }
{ \__juliandate_use:n { db } - \__juliandate_use:n { a } * 365 }
\__juliandate_set:nn { y }
{
\__juliandate_use:n { g } * 400 +
\__juliandate_use:n { c } * 100 +
\__juliandate_use:n { b } * 4 +
\__juliandate_use:n { a }
}
\__juliandate_set:nn { m }
{ \int_div_truncate:nn { \__juliandate_use:n { da } * 5 + 308 } { 153 } - 2 }
\__juliandate_set:nn { d }
{ \__juliandate_use:n { da } - \int_div_truncate:nn { (\__juliandate_use:n { m } + 4) * 153 } { 5 } + 122 }
\int_set:Nn \l_juliandate_year_int
{ \__juliandate_use:n { y } - 4800 + \int_div_truncate:nn { \__juliandate_use:n { m } + 2 } { 12 } }
\int_set:Nn \l_juliandate_month_int
{ \int_mod:nn { \__juliandate_use:n { m } + 2 } { 12 } + 1 }
\int_set:Nn \l_juliandate_day_int
{ \__juliandate_use:n { d } + 1 }
}
\cs_generate_variant:Nn \juliandate_reverse:n { x }

\NewDocumentCommand{\showday}{ m }
{
\juliandate_reverse:n { #1 }
\int_to_arabic:n { \l_juliandate_day_int }-
\int_to_arabic:n { \l_juliandate_month_int }-
\int_to_arabic:n { \l_juliandate_year_int }
}

\NewDocumentCommand{\tomorrow}{ }
{
\group_begin:
\juliandate_reverse:x { \juliandate_calc:nnnn { \day + 1 } { \month } { \year } { \use:n } }
\day = \l_juliandate_day_int
\month = \l_juliandate_month_int
\year = \l_juliandate_year_int
\today
\group_end:
}
\NewDocumentCommand{\tomorrowof}{ m m m }
{
\group_begin:
\juliandate_reverse:x { \juliandate_calc:nnnn { #1 + 1 } { #2 } { #3 } { \use:n } }
\day = \l_juliandate_day_int
\month = \l_juliandate_month_int
\year = \l_juliandate_year_int
\today
\group_end:
}
\ExplSyntaxOff
\begin{document}
\juliandate{18}{12}{2012}

\storejuliandate*{\x}{18}{12}{2012}\x

\storejuliandate*{\x}{1}{1}{1900}\x

\showday{2456280}

\showday{2415021}

\tomorrow

\tomorrowof{31}{12}{2012}

\tomorrowof{28}{2}{2012}

\tomorrowof{29}{2}{2012}

\tomorrowof{28}{2}{2013}

\tomorrowof{28}{2}{1900}

\end{document}

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That is great work on the arithmetic. The problem, as I stated above, is to make it look like what would have been produced by today, which is affected by the documentclass, babel and a few other packages. –  Max Dec 18 '12 at 0:36
@Max so the \tomorrow should not take any arguments. You want a command which uses \today and adds a day, without having to specify which day it is now, is that right? –  Vivi Dec 18 '12 at 0:45
Yes, ideally. This would be accomplished by \def\newtomorrow{\tomorrow{\day}{\month}{\year} already, but it would be wrong for a german, french or american document. While it's trivial to adjust the tomorrow macro on a per user/country/document basis, the solution i am looking for would work with every documentclass and combination of packages. This may not be possible though. –  Max Dec 18 '12 at 0:55
@Max I've shown how you can integrate the macros with babel or datetime –  egreg Dec 18 '12 at 7:49
yes thank you very much. I am having trouble compiling it though. I get ! Command \prop_get:Nn' not yet defined! on line l.72 \cs_generate_variant:Nn \prop_get:Nn I haven't really done much with latex3/expl3 yet and started reading the code for a couple of hours. I only seem to have \prop_get:NnN which isn't consistent with your line i think. This is under texlive 2012, do i need an updated package? –  Max Dec 18 '12 at 14:31

Just a quick cook-up

\documentclass{article}
\usepackage{pgfkeys,pgfcalendar}

\newcount\pgfdatecount
\newcommand{\tomorrow}{%
\pgfcalendardatetojulian{\year-\month-\day+1}{\pgfdatecount}
\pgfcalendarjuliantodate{\the\pgfdatecount}{\myyear}{\mymonth}{\myday}
\pgfcalendarmonthname{\mymonth}\space\myday,\space\myyear%
}

\begin{document}

\today~ is the day before tomorrow.\par
\tomorrow~ is the day after today.

\end{document}
`

Adding weekday names etc. is also possible. You can check the manual. This is, as far as I can paranoi(?!), independent from the class but the month names are fixed. If you like you can introduce them too but ISO dates rulaz just because of this.

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