ISO Date Format

Question

I have done a search on this forum and on the internet and have not found the answer to my question. I am looking to use an ISO date format such as 2018W083 for today (2018-02-21) in my reports. I use the isodate package now for my dates, \usepackage[iso]{isodate}.

If this is a duplicate question, please let me know. I could not find it.

Answer 1

This is (surprisingly) not trivial. There is Macro for week number but the implementation there seems to be slightly unreliable. If you are willing to use LuaLaTeX then you can make a system call to find the week number (and other parts of the date).

Note that the system library defines Sunday as 0, while ISO 8601 defines it as 7.

MWE:

\documentclass{article}
\usepackage{luacode}
\newcommand{\weektoday}{\printdate{\the\year}{\the\month}{\the\day}}
\newcommand{\printdate}[3]{%
\luaexec{
timestamp = os.time{year=#1, month=#2, day=#3}
daynr = os.date("\%w",timestamp)
if daynr == "0" then daynr = "7" end
tex.sprint(os.date("\%YW\%V-",timestamp)..daynr)
}
}
\begin{document}
Today: \weektoday

Other date: \printdate{2018}{3}{4}
\end{document}

Result:

---

Compatibility note: on Windows the %V format is not implemented. Instead you can use %U(first Sunday starts week 0) or %W(first Monday starts week 0), however both are different from the ISO standard (first Thursday determines week 1). You can calculate the ISO week number manually in Lua but in that case you may just as well use a pure LaTeX solution.