Date Timezone Oracle PLSQL

SQL> SELECT LAST_DAY('20-JUN-99') "Last_Day",
  2            LAST_DAY('20-JUN-99') - TO_DATE('20-JUN-99') "Days_Summer"
  3       from DUAL;
Last_Day             Days_Summer
-------------------- -----------
30-JUN-0099 00:00:00          10
1 row selected.
SQL>
SQL>
SQL> --