#include <iostream>
#include <ctime>
#include <string>
#include <sstream>
#include <stdexcept>
// #include <sys/time.h>

// invariant: time_str contains today's local wall clock time in the form 'hh:mm:ss'
std::time_t str_to_time_t( const std::string& time_str )
{
    constexpr char COLON = ':' ;
    enum { UB_HR = 24, UB_MIN = 60, UB_SEC = 60 };

    char delimiter ;
    const std::time_t now = std::time(nullptr) ;
    std::tm tm = *std::localtime( &now );

    std::istringstream stm(time_str) ;
    if( ( stm >> tm.tm_hour && tm.tm_hour >= 0 && tm.tm_hour < UB_HR ) &&
        ( stm >> delimiter && delimiter == COLON ) &&
        ( stm >> tm.tm_min && tm.tm_min >= 0 && tm.tm_min < UB_MIN ) &&
        ( stm >> delimiter && delimiter == COLON ) &&
        ( stm >> tm.tm_sec && tm.tm_sec >= 0 && tm.tm_sec < UB_SEC ) &&
        ( stm >> std::ws && stm.eof() ) )
    {
        return std::mktime(&tm) ;
    }
    else throw std::invalid_argument( "invalid time string" ) ;
}

int main() // minimal test driver
{
    const std::string test[] = { "2:34:6", "19:74:55", "12x18y0", "23:59:59", "12:0:0" } ;
    for( const std::string& s : test )
    {
        try
        {
            const std::time_t time_to_set = str_to_time_t(s) ;
            const std::tm tm = *std::localtime( &time_to_set );
            char cstr[128] ;
            std::strftime( cstr, sizeof(cstr), "%A, %Y %B %d %H:%M:%S", &tm ) ;
            std::cout << cstr << " (" << time_to_set << ")\n" ;
            // const time_val tv { time_to_set, 0 } ;
            // settimeofday( &t, nullptr ) ;
        }
        catch( const std::exception& )
        {
            std::cerr << "badly formed time string '" << s << "'\n" ;
        }
    }
}