create or replace procedure output_dates 
as 
  l_sysdate date := sysdate; 
  l_dbtimezone varchar2(16) := dbtimezone; 
  l_curdate date := current_date; 
  l_sessiontimezone varchar2(16) := sessiontimezone; 
  begin 
    dbms_output.PUT_LINE( 
      'Sysdate (' || l_dbtimezone || '): ' 
      || to_char(l_sysdate, 'HH24:MI') 
      || ', Current_Date (' || l_sessiontimezone || '): ' 
      || to_char(l_curdate, 'HH24:MI') 
      || ', Difference (in hours): ' 
      || to_char((l_curdate-l_sysdate)*24)); 
  end; 

alter session set time_zone = '-6:00'

call output_dates()

alter session set time_zone = '+2:00'

call output_dates()