Convert string to number

Question

How can I convert numbers given as strings to "real" numbers? Consider the following MWE:

\documentclass{article}
\usepackage{datenumber}
\usepackage{xstring}
\begin{document}
\newcommand{\tag}{\StrBefore{01.01.2000}{.}}
%\setdatenumber{2000}{10}{\tag}
\setdatenumber{2000}{10}{10}
\tag \\
\datedate
\end{document}

Here, \tag is set to 01 as can be seen in the output. But if I use the line \setdatenumber{2000}{10}{\tag} I get the error "missing number, treated as zero". How can I avoid this? None of the other "Convert string to number" I found questions seem to be of help here.

Answer 1

The problem is caused by \StrBefore not being expandable, so the number is printed but it can not be used for further calculations. The xstring author added a optional argument to \StrBefore to help with this kind of problems: You can pass the name of a control sequence in [] to set this control sequence to the result of \StrBefore:

\documentclass{article}
\usepackage{datenumber}
\usepackage{xstring}
\begin{document}
\StrBefore{01.01.2000}{.}[\tag]
\setdatenumber{2000}{10}{\tag}
\tag \\
\datedate
\end{document}