The newtzset.3 man page says The tzalloc function allocates and returns a timezone object described by TZ. If TZ is not a valid timezone description, or if the object cannot be allocated, tzalloc returns a null pointer and sets errno. This doesn't explicitly say that TZ == NULL amounts to "not a valid timezone description", nor does it explicitly say that implementation-defined default timezone information is used. (The latter is the behavior the code implements.) It does say If TZ is null, the best available approximation to local (wall clock) time, as specified by the tzfile(5)-format file localtime in the system time conversion information directory, is used. If TZ is the empty string, UT is used, with the abbreviation "UTC" and without leap second correction; please see newctime(3) for more about UT, UTC, and leap seconds. If TZ is nonnull and nonempty: if the value begins with a colon, it is used as a pathname of a file from which to read the time conversion information; if the value does not begin with a colon, it is first used as the pathname of a file from which to read the time conversion information, and, if that file cannot be read, is used directly as a specification of the time conversion information. after describing taser: The tzset function acts like tzalloc(getenv("TZ")), except it saves any resulting timezone object into internal storage that is accessed by localtime, localtime_r, and mktime. The anonymous shared timezone object is freed by the next call to tzset. If the implied call to tzalloc fails, tzset falls back on Universal Time (UT). but it might be clearer if the paragraph about tzset followed the "If TZ is..." text. Perhaps the "If TZ is not a valid timezone description, or..." text should immediacy follow the "If TZ is..." text, e.g. The tzalloc function allocates and returns a timezone object described by TZ. If TZ is null, the best available approximation to local (wall clock) time, as specified by the tzfile(5)-format file localtime in the system time conversion information directory, is used. If TZ is the empty string, UT is used, with the abbreviation "UTC" and without leap second correction; please see newctime(3) for more about UT, UTC, and leap seconds. If TZ is nonnull and nonempty: if the value begins with a colon, it is used as a pathname of a file from which to read the time conversion information; if the value does not begin with a colon, it is first used as the pathname of a file from which to read the time conversion information, and, if that file cannot be read, is used directly as a specification of the time conversion information. In the latter case, if TZ is not a valid timezone description, tzalloc returns a null pointer and sets errno. If the object cannot be allocated, or if a file cannot be read, tzalloc returns a null pointer and sets errno. The tzset function acts like tzalloc(getenv("TZ")), except it saves any resulting timezone object into internal storage that is accessed by localtime, localtime_r, and mktime. The anonymous shared timezone object is freed by the next call to tzset. If the implied call to tzalloc fails, tzset falls back on Universal Time (UT). with, if necessary, text reminding people that, if TZ is not seen in the environment, tzalloc(getenv("TZ")) is equivalent to tzalloc(NULL), so "the best available approximation to local (wall clock) time..." is used.