mktime returns -1 but valid data

Question

I try to find a way for a default date (if date is not valid). Common way works fine:

set(2022,6,17,12,12,0,0,0)

void KTime::Set(int nYear, int nMonth, int nDay, int nHour, int nMin, int nSec, int nDST, bool isUTC)
{
    assert(nYear >= 1900);
    assert(nMonth >= 1 && nMonth <= 12);
    assert(nDay >= 1 && nDay <= 31);
    assert(nHour >= 0 && nHour <= 23);
    assert(nMin >= 0 && nMin <= 59);
    assert(nSec >= 0 && nSec <= 59);

    struct tm atm;
    atm.tm_sec = nSec;
    atm.tm_min = nMin;
    atm.tm_hour = nHour;
    atm.tm_mday = nDay;
    atm.tm_mon = nMonth - 1;        // tm_mon is 0 based
    atm.tm_year = nYear - 1900;     // tm_year is 1900 based
    atm.tm_isdst = nDST;
    m_time = isUTC ? utc_mktime(&atm) : mktime(&atm);

    assert(m_time != -1);       // indicates an illegal input time
}

But if I set to the same function:

set(1900,1,1,0,0,0,0,0)

I will get a mktime = -1 Any idea where is my logic bomb?

Answer 1

mktime (and the rest of the POSIX date functions) only work for dates >= 1970-01-01 00:00:00, the UNIX epoch.

mktime, quoth the manual,

returns -1 if time cannot be represented as a time_t object

and 1900 definitely can't be represented as a time_t, since it's 70 years early.