\begin{table}%t2
  \caption{Summary of the observation periods.}
  \label{tab:obs_period}
\setlength{\tabcolsep}{10pt}
\renewcommand{\arraystretch}{1.1}
%%\centering
\par
     \begin{tabular}{lll}
        \hline
        \hline
        Observation Date & $T_{{\rm on}}$(min) & $T_{{\rm off}}$(min)\\
        \hline
        03-Oct. -- 18-Nov. 2000 & 2297 & 2245\\
        20-Sep. -- 15-Nov. 2001 & 2567 & 2401\\
        \hline
        Total & 4864 & 4646\\
        \hline
     \end{tabular}
\end{table}