[bitpow]

Is this accurate though? Isn’t the number of seconds since 1970 absolute, and it’s up to the library generating the Gregorian date to take into account leap seconds? I suppose if these libraries are not taking into account leap seconds, then the actual rollover will be a few seconds earlier (or later?) than what we think.

[fanf2]

See the definition of seconds since the epoch at https://pubs.opengroup.org/onlinepubs/9699919799.2018edition...

  tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 +
      (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 -
      ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400

[JdeBP]

Your time_t isn't the true number of SI seconds since the Epoch. It's that number, mucked around 27 times so far. Instead of leaving their kernels to count one tick per SI second (ignoring oscillator drift) many people slew, step, or smear their seconds count every time that there is a leap second.

Go and read about "right" versus "posix" timezones.

* https://unix.stackexchange.com/a/294715/5132