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use Modern::Perl;
use Date::Calc qw/ Date_to_Time Time_to_Date /;
...
      : sprintf '%4d-%02d-%02d %02d:%02d:%02d for %02d minutes',
      Time_to_Date( $tStart ), ( $tEnd - $tStart ) / 60;
}

2012-08-17 12:00:00 for 30 minutes