Oracle - Performance - Timing a Statement

Always, start with a fresh database session to remove any caching from previous runs.

connect user/password@database;

set timing on;

select 
	PKG_ACCT_DECRYPTION_FUNCTIONS.f_secure_decrypt(PKG_ACCT_ENCRYPTION_FUNCTIONS.f_secure_encrypt(to_char(sysdate,'mm/dd/yyyy hh24:mi:ss')))
from dual;

Also see Explain Plan and SysTimeStamp with Milliseconds.