Theatrical-to-DVD windows for the first half of 2009 are more than two weeks longer than they were early last year, a phenomenon industry execs are attributing to this year’s late Easter and the unusually strong box office business.

For all theatrical titles released on or announced for DVD since January, the average window is 144 days, according to Rentrak research. That compares with 125 days for the same frame last year and 133 for full year 2008.

For theatrical titles that earned at least $10 million at the box office, the average window shrinks to 129 days, but that’s still longer than the 120-day window in the first half of 2008 for the same sort of films.

The subject of windows’ duration is traditionally a controversial one. Studios must walk a fine line between the competing interests of theaters, which want films to run as long as possible on their screens, and retailers, who want the DVDs on their shelves as fast as possible. Studio execs declined to comment publicly about the forces behind this year’s longer windows.

One widely held belief is that a number of studios simply waited for films to hit the street around Easter, which this year fell on April 12, three weeks later than in 2008, when the holiday was on March 23.

“I don’t think there is a whole lot of change in how people are slating DVDs,” said one executive. “I think this really has something to do with seasonality.”

However, most of the April 7 releases (the closest Tuesday to Easter) came with relatively short windows, such as Walt Disney Studios Home Entertainment’s “Bedtime Stories” and Warner Home Video’s “Yes Man” at 103 and 111 days, respectively.

Another explanation is that certain films were granted longer theatrical berths because demand was high to see them on the bigscreen. Warner’s June 9 release “Gran Torino,” with its 179-day window, was a surprise box office hit, grossing $148 million in the U.S.

This year has also ushered in increasing numbers of 3-D films, which have generally proved popular on the bigscreen. In addition, 3-D pictures earn studios and theaters higher ticket revenues than comparable 2-D films, arguably helping them stay put in theaters.

A spokesman for the National Assn. of Theater Owners said that while “no 3-D movie has had a terribly short window,” many of these films’ bigscreen runs are dependent on when a subsequent 3-D movie is released. There are a limited number of 3-D-capable screens, so one pic’s run typically ends when another 3-D film comes along.

The window for 3-D film “Coraline,” due on DVD/Blu-ray on July 21 from Universal Studios Home Entertainment, is a long 165 days. Lionsgate’s May 19 DVD/Blu-ray title “My Bloody Valentine 3-D” has a relatively long window for the studio at 123 days. Many of its titles’ windows clock in at around 100 days or less.

One studio executive argued that “it’s not so much tied to the 3-D of it all as it is how long of legs any given film has at the box office, whether it’s in 2-D or 3-D. We give a film every opportunity to extend its life in theaters, if there’s still a demand for it, before we choose a DVD street date.”

Studios insist they haven’t significantly altered their DVD slotting procedures in 2009. The companies still look for those dates that will make titles successful.

“There is no change in a strategy for windows,” said one studio source. “We are just trying to provide the best opportunity to sell DVDs.”

(Susanne Ault is a reporter for Daily Variety sister publication Video Business.)