Without using titlesec
, you can redefine the \@makechapterhead
command (defined in book.cls
) which actually typesets the chapter title; you could do something along these lines:
\documentclass{book}
\usepackage{lipsum}
\newcommand\toptitle{}
\makeatletter
\def\@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\toptitle\par
\huge\bfseries \@chapapp\space \thechapter
\par\nobreak
\vskip 20\p@
\fi
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}
\makeatother
\begin{document}
\renewcommand\toptitle{{\itshape Quantum Mechanics\hfill 15.10.2011}}
\chapter{Test chapter}
\lipsum
\end{document}

Using the etoolbox
package, the above redefinition can be done in a shorter way:
\documentclass{book}
\usepackage{etoolbox}
\usepackage{lipsum}
\newcommand\toptitle{}
\makeatletter
\patchcmd{\@makechapterhead}{\if@mainmatter}{\if@mainmatter\toptitle\par}{}{}
\makeatother
\begin{document}
\renewcommand\toptitle{{\itshape Quantum Mechanics\hfill 15.10.2011}}
\chapter{Test chapter}
\lipsum
\end{document}