Get timezone offset with cache optimization.

timezone.cpp

#include <time.h>
#include <iostream>
#include <limits>

int TimeZoneOffsetSecond()
{
    // cache, need compute only once
    static int offset = std::numeric_limits<int>::min();
    if (offset != std::numeric_limits<int>::min())
    {
        return offset;
    }
    time_t t = time(NULL);
    struct tm utc;
    gmtime_r(&t, &utc);
    offset = t - mktime(&utc);
    return offset;
}

int main()
{
    // test case 1: 32400
    //  TZ=:/usr/share/zoneinfo/Asia/Tokyo ./a.out
    // test case 2: 28800
    //  TZ=:/usr/share/zoneinfo/Asia/Shanghai ./a.out
    // test case 3: 10800
    //  TZ=:/usr/share/zoneinfo/Africa/Kampala ./a.out
    // test case 4: -18000
    //  TZ=:/usr/share/zoneinfo/America/Indiana/Indianapolis ./a.out
    std::cout << TimeZoneOffsetSecond() << std::endl;
}