Yes, you can wear dress pants and a glitzy shirt. Formal night wear ranges from casual wear (no jeans or t shirts) to true Tux and Evening Gown formal wear. It depends on what you want to do. If you don't have a formal dress, I wouldn't go buy one unless that is what you want to wear.
A different question.. if you are leaving on the 30th, was does your cruise ticker say June of next year?
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013