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?

KTime::Set is definitely not C, and then you can set which isn't a valid function here. But in C++ why don't just use std::chrono? — phuclv
– phuclv, Commented Jun 18, 2022 at 9:32
KTIme is an own class for time handling. This function is just to validate the given date, not to fill any values or parameter. — IFThenElse
– IFThenElse, Commented Jun 18, 2022 at 11:23
yes, std::chrono is the Swiss-knife for date/time handling and it contains everything, no need to resort to ancient C tools. The code would be much shorter — phuclv
– phuclv, Commented Jun 18, 2022 at 11:34

AKX · Accepted Answer · 2022-06-18 09:10:15Z

6

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.

answered Jun 18, 2022 at 9:10

AKX

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2 Comments

phuclv Over a year ago

The GNU C Library additionally guarantees that time_t is a signed type, and that all of its functions operate correctly on negative time_t values, which are interpreted as times before the epoch. so if you use the GNU libc then it should work

IFThenElse Over a year ago

Ok, this 1971 (mktime only accepts 1971) magic was not known. On Windows it is common 1900. Thank you for that hint.

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