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


\def\timediffcalc(#1:#2-#3:#4){%
  \FPeval\btm{#1 * 60 + #2}
  \FPeval\etm{#3 * 60 + #4}
  \FPeval\adtm{\etm - \btm}
  \FPeval\cdtm{1440 + \etm - \btm}
  \FPifpos\adtm
    \FPset\dtm{\adtm}
  \else
    \FPset\dtm{\cdtm}
  \fi
  \FPeval\dth{\dtm / 60}
  \FPtrunc\dthtrunc\dth2}

\def\timediffprn(#1:#2-#3:#4){%
 \timediffcalc(#1:#2-#3:#4)
 \dthtrunc\relax}


\def\newtime#1(#2:#3-#4:#5){%
  \timediffcalc(#2:#3-#4:#5)
  \expandafter\let\csname #1\endcsname=\dthtrunc}



\begin{document}


\timediffprn(10:15-16:00) 

\timediffprn(10:15-01:00) 

\timediffprn(9:15-01:20) 

\vspace{1cm}


\newtime{first}(10:15-16:00)
\newtime{second}(10:15-01:00)
\newtime{anyother}(9:15-01:20)

\first

\second

\anyother




\begin{invoice}{EUR}{0}
	\ProjectTitle{Rechnung }%
		%    Contents     Rate/Unit    Count
		\Fee{ab.cd.de}     {12.5}   {\first}
		\Fee{ab.cd.de}     {12.5}   {\second}
		\Fee{ab.cd.de}     {12.5}   {\anyother}
	\ProjectTitle{Sales Restructuring}%
		%    Contents                    Rate/Unit Count
		\Fee{Sales Structure Analysis}   {2750.00} { 6}
		\Fee{Negotiation with Agents}    {2750.00} { 3}
\end{invoice}







\end{document}



