\documentclass{report}
\usepackage[utf8]{vietnam}
\newcommand{\thoigian}{\def \@tg}
\thoigian{2h}%
\begin{document}
Bay gio la: \@tg
\end{document}
\documentclass{report}
\usepackage[utf8]{vietnam}
\newcommand{\thoigian}{\def \@tg}
\newcommand{\ngay}{\def \@ng}
\thoigian{2h}
\ngay{22}
\begin{document}
Thoi gian: \@tg
Ngay: \@ng
\end{document}