std::localtime one second leads to 30 mins gap

Question

#include <iostream>
int main(){
    std::time_t t = 893665799;
    std::tm * tm = std::localtime(&t);
    printf("local time duration =>year:%d, month:%d, day:%d, hour:%d, min:%d, sec:%d\n",tm->tm_year, tm->tm_mon, tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec);

    t = 893665800;
    tm = std::localtime(&t);
    printf("local time duration =>year:%d, month:%d, day:%d, hour:%d, min:%d, sec:%d\n",tm->tm_year, tm->tm_mon, tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec);
}

893665799 is a magic utc.

When u change ur timezone to Pitcairn Islands.

The log shows below

local time duration =>year:98, month:3, day:26, hour:23, min:59, sec:59

local time duration =>year:98, month:3, day:27, hour:0, min:30, sec:0

what's going on ? why 1 sec leads to 30 mins gap??

Answer 1

Given that the time_t given is exactly one second prior to rolling into a new hour, it's almost certainly to do with daylight savings time or some other adjustment.

And, with a bit of Google-fu, here it is:

27 Apr 1998 - Time Zone Change (PNT → PST)

When local standard time was about to reach Monday, 27 April 1998, 12:00:00, midnight clocks were turned forward 0:30 hours to Monday, 27 April 1998, 12:30:00 am local standard time instead.

This actually happens quite often, with various countries around the world moving their local times willy-nilly all over the place :-)