Oralce PL/SQL : Creating calendar table

SQL> create table calendar(c_date date);

Table created.

SQL> insert into calendar
  2  select to_date(to_char(19900101), 'yyyymmdd') + lvl d
  3  from ( select level -1 lvl
  4  from dual
  5  connect by level <= 20000);

20000 rows created.