OS/400 does not have tzname.  Code in ext/standard/datetime.c does not check to see if HAVE_TZNAME is defined before accessing the field.  Here is the code:

#if HAVE_TM_ZONE
                                size += strlen(ta->tm_zone);
#else
                                size += strlen(tzname[0]);
#endif

and

#if HAVE_TM_ZONE
                                strcat(return_value->value.str.val, ta->tm_zone);
#else
                                strcat(return_value->value.str.val, tzname[0]);
#endif

It may be that for OS/400 we may need to do something different, but in the meantime HAVE_TZNAME needs to be checked prior to accessing tzname.