Tropic Thunder is a 2008 black satirical action comedy film directed by Ben Stiller, who stars with Jack Black and Robert Downey Jr. as a group of prima donna actors who are making a fictional Vietnam War film. When their frustrated director drops them in the middle of a jungle, they are forced to rely on their acting skills to survive the real action and danger. The screenplay by Justin Theroux, Stiller, and Etan Cohen was from a story by Stiller and Theroux. The film was produced by Stuart Cornfeld, Stiller and Eric McLeod for Red Hour Productions and DreamWorks Pictures as an international co-production between the United States, Germany, and the United Kingdom.

Although Southern California and Mexico were considered for the main unit filming, the Hawaiian island of Kaua'i (where Stiller has a home) was selected for the majority of the shooting. Kaua'i was chosen over Mexico because a tax credit for in-state spending was negotiated with the Kaua'i Film Commission. John Toll, the cinematographer, stated the island was also selected for its similarity to Vietnam, based on its dense foliage, variety of terrains, and weather. Kaua'i was first scouted as a possible location to film Tropic Thunder in 2004. Stiller spent more than 25 hours over 6 weeks exploring the island, using all-terrain vehicles, boats, and helicopters. After the film was greenlit by DreamWorks in 2006, pre-production lasted for six months, most of this time spent on scouting additional locations for filming. Filming for the Los Angeles and interior scenes occurred on sets at Universal Studios in Hollywood.  Tropic Thunder was the first major studio production on Kaua'i in five years. After filming was completed, it was deemed the largest production filmed on the island to date, and contributed more than $60 million to the local economy. Tim Ryan, the executive editor of Hawaii Film & Video Magazine, commented on the filming on the island: "I think Tropic Thunder will give Kaua'i much needed and long idled publicity in the production arena ... It should put Kaua'i back on the production consideration radar." Preliminary production crews were on the island starting in December 2006 and principal photography began in July 2007, with filming lasting thirteen weeks over seven separate locations on the island. Much of the filming took place on private land as well as conservation status designated areas. Casting calls on the island sought 500 residents to portray the villagers in the film. Two units shot simultaneously on the island from the ground, and an aerial unit shot from helicopters. Many of the sets and the bridge used for one of the final scenes were built in three months. The island's erratic weather hampered filming with rain and lighting issues. The crew also faced complications in moving the equipment and cast due to the difficult terrain.  The film advising company Warriors Inc. was enlisted to ensure the war scenes, including the attire worn by the actors, looked authentic. Former members of the U.S. military taught the actors how to handle, fire, and reload their weapons, as well as perform various tactical movements. The opening war scene was filmed over three weeks and required fifty stuntmen. Animatics were used to map out the necessary camera angles for filming.

Answer the following question by taking a quote from the article: When was the movie filmed?
Preliminary production crews were on the island starting in December 2006 and principal photography began in July 2007, with filming lasting thirteen weeks