Converting time_t to int

You should cast it to a long int instead of int.

long int t = static_cast<long int> (time(NULL));

An int might not be enough to hold the time, for example, on my platform, time_t is a typedef of __int64.

Whatever you're doing with time_t, you'll probably be best off using the <chrono> library. Then you can convert to and from time_t, if necessary. Hopefully you can just do what you need in <chrono>

#include <chrono>

int main() {
    auto a = std::chrono::system_clock::now()
    time_t b = std::chrono::system_clock::to_time_t(a);
    auto c = std::chrono::system_clock::from_time_t(b);
}

Update:

The C++ standard library doesn't yet have an API for dates as good as <chrono> is for time. You can use the Boost date library or you can fall back on the C date library:

#include <ctime>
#include <chrono>
#include <iostream>

int main() {
    std::tm epoch_start = {};
    epoch_start.tm_sec = 0;
    epoch_start.tm_min = 0;
    epoch_start.tm_hour = 0;
    epoch_start.tm_mday = 1;
    epoch_start.tm_mon = 0;
    epoch_start.tm_year = 70;
    epoch_start.tm_wday = 4;
    epoch_start.tm_yday = 0;
    epoch_start.tm_isdst = -1;

    std::time_t base = std::mktime(&epoch_start);

    auto diff = std::chrono::system_clock::now() - std::chrono::system_clock::from_time_t(base);
    std::chrono::seconds s = std::chrono::duration_cast<std::chrono::seconds>(diff);

    std::cout << s.count() << '\n';
}