Нижнее подчёркивание пустой строки

\underline{\hspace{3cm}}

\makebox[1cm]{\hrulefill}

\makeatletter
  \def\vhrulefill#1{\leavevmode\leaders\hrule\@height#1\hfill \kern\z@}
\makeatother
\newcommand{\signform}[1]%
{\vhrulefill{0.75pt}~{#1}}  

%% date form pattern
\newcommand{\dateform}%
{<<\rule{2em}{0.75pt}>>~\rule{6em}{0.75pt}~2013~г.}

%% person form
\newcommand{\personform}[1]%
{\signform{#1}\\\dateform}