#include "boost/date_time/posix_time/posix_time_types.hpp"
#include "boost/date_time/posix_time/conversion.hpp"

int main( int argc, char *argv[] )
{
   time_t test = 1274047200;
   boost::posix_time::ptime boost_time = boost::posix_time::from_time_t( test );
   std::tm tm = boost::posix_time::to_tm( boost_time );

   //а вот тут внимание: 1274036400
   time_t back = mktime(&tm);
   return 0;
}