How to convert a UTC date & time to a time_t in C++?

Question

I want to convert a UTC date & time given in numbers for year, month, day, etc. to a time_t. Some systems offer functions like mkgmtime or timegm

Answer 1

For completeness, here's a version of mkgmtime() that takes a struct tm* as argument:

static time_t mkgmtime(const struct tm *ptm) {
    time_t secs = 0;
    // tm_year is years since 1900
    int year = ptm->tm_year + 1900;
    for (int y = 1970; y < year; ++y) {
        secs += (IsLeapYear(y)? 366: 365) * SecondsPerDay;
    }
    // tm_mon is month from 0..11
    for (int m = 0; m < ptm->tm_mon; ++m) {
        secs += DaysOfMonth[m] * SecondsPerDay;
        if (m == 1 && IsLeapYear(year)) secs += SecondsPerDay;
    }
    secs += (ptm->tm_mday - 1) * SecondsPerDay;
    secs += ptm->tm_hour       * SecondsPerHour;
    secs += ptm->tm_min        * SecondsPerMinute;
    secs += ptm->tm_sec;
    return secs;
}