C - mktime altering passed time_t value

Question

I need the functionality for a user to enter a timestamp, store that timestamp on the disk, then display that timestamp at a later time. I also need to be able to determine the distance between two timestamps.

My current solution is to use the time.h utility as follows:

To read a given timestamp of the form "MO/DA/YEAR HR:MI"

time_t readable_to_epoch(char* str) {
    int mo, da, yr, hr, mi;
    sscanf(str, "%d/%d/%d %d:%d", &mo, &da, &yr, &hr, &mi);
    
    struct tm timeinfo;
    time_t out;
    timeinfo.tm_mon = mo - 1;
    timeinfo.tm_mday = da;
    timeinfo.tm_year = yr - 1900;
    timeinfo.tm_hour = hr;
    timeinfo.tm_min = mi;
    timeinfo.tm_sec = 0;

    out = mktime(&timeinfo);
    return out;
}

To convert a given time_t into a human readable form:

// WARNING returned string must be freed;
char* epoch_to_readable(time_t time) {
    return asctime(localtime(&time));
}

My issue is that if I pass in the input: 06/19/2018 12:52 The conversion to time_t and back is as desired. Here is the test harness output:

original: 06/19/2018 12:52
epoch time: 1529437920
converted readable: Tue Jun 19 12:52:00 2018

But when I pass in the input: 01/01/2022 10:00 The converted readable seems to be one hour behind:

original: 01/01/2022 10:00
epoch time: 1641056400
converted readable: Sat Jan  1 09:00:00 2022

I have no idea why this is happening, and google isn't helping. What do I need to do to fix this?

Answer 1

You did not set timeinfo.tm_isdst, so you're getting undefined behaviour. You should use -1.

Daylight Saving Time flag. The value is positive if DST is in effect, zero if not and negative if no information is available

Note that working with local times without offset is inherently buggy. Because of Daylight Saving Time, time stamps from two hours each year are indistinguishable from each other.