@justinmk `man localtime` gives

> The functions ctime(), gmtime(), and localtime() all take as an argument a time value representing the time in  seconds since the Epoch (00:00:00 UTC, January 1, 1970; see time(3)).  When encountering an error, these functions return NULL and set errno to an appropriate value.

This comment work as a note that one of the possible error conditions is an invalid value of seconds.