\documentclass[11pt,twoside,a4paper]{article}
\usepackage{a4wide,amsmath,amssymb}
\usepackage{invoice}

\usepackage{ifthen}
\usepackage{calc}

\newcounter{bh}
\newcounter{bm}
\newcounter{eh}
\newcounter{em}
\newcounter{dh}
\newcounter{dm}
\newcounter{hrdec}


\def\timediff#1:#2-#3:#4 {%
  \setcounter{bh}{#1}
  \setcounter{bm}{#2}
  \setcounter{eh}{#3}
  \setcounter{em}{#4}
  \ifthenelse{\(\value{eh}>\value{bh}\or\value{eh}=\value{bh}\)
              \and\(\value{em}>\value{bm}\or\value{em}=\value{bm}\)}
    {\setcounter{dh}{\value{eh}-\value{bh}}
     \setcounter{dm}{\value{em}-\value{bm}}}
    {\ifthenelse{\(\value{eh}>\value{bh}\or\value{eh}=\value{bh}\)
                 \and\value{em}<\value{bm}}
      {\setcounter{dh}{\value{eh}-\value{bh}-1}
       \setcounter{dm}{60+\value{em}-\value{bm}}}
      {\ifthenelse{\value{eh}<\value{bh}
        \and\(\value{em}>\value{bm}\or\value{em}=\value{bm}\)}
       {\setcounter{dh}{24+\value{eh}-\value{bh}}
        \setcounter{dm}{\value{em}-\value{bm}}}
       {\ifthenelse{\value{eh}<\value{bh}\and\value{em}<\value{bm}}
       {\setcounter{dh}{23+\value{eh}-\value{bh}}
        \setcounter{dm}{60+\value{em}-\value{bm}}}}}{}}
    \setcounter{hrdec}{\value{dm}*5/3}
    \ifthenelse{\value{dm}>5}{\thedh.\thehrdec}{\thedh.0\thehrdec}}


\begin{document}


\timediff10:15-16:00

\begin{invoice}{EUR}{0}
        \ProjectTitle{Rechnung }%
                %    Contents                    Rate/Unit Count
\edef\foo{{ab.cd.de}{12.5}{\timediff10:15-16:00}} \expandafter\Fee\foo
%                \Fee{ab.cd.de}     {12.5}   {\timediff10:15-16:00}
%                \Fee{ab.cd.de}     {12.5}   {10}
        \ProjectTitle{Sales Restructuring}%
                %    Contents                    Rate/Unit Count
                \Fee{Sales Structure Analysis}   {2750.00} { 6}
                \Fee{Negotiation with Agents}    {2750.00} { 3}
\end{ivoice}
\end{document}





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`Real arithmetic', Version 1.0, Jan. 1993(C) Frank Buchholz)
(/usr/share/texmf-texlive/tex/latex/invoice/invoice.def)) (./invoice_test.aux)


VAT is zero!

! Argument of \timediff has an extra }.
<inserted text>
                \par
l.49 ...foo{{ab.cd.de}{12.5}{\timediff10:15-16:00}
                                                  } \expandafter\Fee\foo
?