Scheduled user locking or unlocking on LINUX

A little C++ program to unlock a LINUX login on Saturdays afternoons follows. All suggestions are welcome.

#include <cstdlib>
#include <ctime>
#include <sstream>
/// av[0] USERNAME
/// to unlock between 12:00 and 19:59 on Saturdays.
int main(int ac,char** av)
{
   if( 2 == ac )
   {
      const std::time_t now( std::time(NULL) );
      const std::tm* timeinfo = std::localtime( &now );
      std::ostringstream cmd;
      cmd << "/usr/sbin/usermod -";
      if( timeinfo->tm_wday == 6
            && timeinfo->tm_hour > 11
            && timeinfo->tm_hour < 20 )
      {
         cmd << 'U';
      }
      else
      {
         cmd << 'L';
      }
      cmd << ' ' << av[1];
      std::system( cmd.str().c_str() );
      return EXIT_SUCCESS;
   }
   else
   {
      return EXIT_FAILURE;
   }
}