Historical clue formulations

• ___ vivendi — 2011-02-24
• Part of M.O. — 2008-05-06
• The "m" in m.o. — 2001-11-29
• ___ operandi — 1999-08-31
• _____ operandi — 1995-06-06
• Part of M.O. — 1995-01-22