Converting time_t to int

Question

I want to convert a given time into epoch(time_t) and vice versa. can anyone tell what is the routine or algorithm for this?

Thanks

Update

 epoch_strt.tm_sec = 0;
 epoch_strt.tm_min = 0;
 epoch_strt.tm_hour = 0;
epoch_strt.tm_mday = 1;
epoch_strt.tm_mon = 1;
epoch_strt.tm_year = 70;
 epoch_strt.tm_isdst = -1;

double nsecs = difftime(curtime, basetime);//current time from system, I converrting it to struct tm

but this always return 32399 for some reason.

Answer 1

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.

Answer 2

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';
}

Answer 3

To convert a struct tm to a time_t, use mktime. To convert a time_t to a struct tm, use either gmtime or localtime.

Sample:

#include <iostream>
#include <ctime>

int main () {
    std::time_t now = std::time(0);

    std::tm *now_tm = gmtime(&now);

    std::tm tomorrow_tm(*now_tm);

    tomorrow_tm.tm_mday += 1;
    tomorrow_tm.tm_hour = 0;
    tomorrow_tm.tm_min = 0;
    tomorrow_tm.tm_sec = 0;

    std::time_t tomorrow = std::mktime(&tomorrow_tm);

    double delta = std::difftime(tomorrow, now);

    std::cout << "Tomorrow is " << delta << " seconds from now.\n";
}

---

UPDATE 2

#include <iostream>
#include <ctime>
#include <cassert>

// Display the difference between now and 1970-01-01 00:00:00
// On my computer, this printed the value 1330421747
int main () {

    std::tm epoch_strt;
    epoch_strt.tm_sec = 0;
    epoch_strt.tm_min = 0;
    epoch_strt.tm_hour = 0;
    epoch_strt.tm_mday = 1;
    epoch_strt.tm_mon = 0;
    epoch_strt.tm_year = 70;
    epoch_strt.tm_isdst = -1;

    std::time_t basetime = std::mktime(&epoch_strt);

    std::time_t curtime = std::time(0);

    long long nsecs = std::difftime(curtime, basetime);

    std::cout << "Seconds since epoch: " << nsecs << "\n";
    assert(nsecs > 42ll * 365 * 24 * 60 * 60);
}