I'm doing some research on Disney shorts, and I came upon an anomaly. In many common resources, it's said that the Goofy short Double Dribble was released on December 20, 1946, and the Pluto short Pluto's Housewarming on February 21, 1947.

But looking through old trade magazines like Boxoffice, Motion Picture Herald and Harrison's Reports, the release dates for the two shorts are cited as 11/29/46 and 12/20/46, respectively. Even well into 1947, the dates were never revised. Does this mean those are the true release dates of these two shorts?

The introduction to the Disney filmography in Of Mice and Magic (which lists the commonly-given dates) says the following about its release dates:

Silent films are grouped under year of release. There were no release dates for Disney's talkie cartoons until mid-1932; the dates listed here are delivery dates recorded in studio files. The remaining release dates were supplied by Walt Disney Archives and do not always conform with those listed in trade magazines, normally our source for such information.

Are the dates from the Archives more accurate than those from the trade magazines? I don't know, but in lieu of further elaboration or proof I'd say not necessarily. The booking charts in the trades aren't infallible, as evidenced by the release dates sometimes being eventually revised, but they're things the exhibitors themselves were going by back in the day. They're primary sources (or at least, as close to them as you're often going to get), profitable for research. When multiple magazines' charts are in agreement on a date without it ever being revised, that's pretty compelling evidence.

Of course, it's always good to remember that even when accurately reported, a short's official "release date" isn't necessarily actually the date it was first exhibited in theaters; if I'm not misremembering, it's more of a "should be available at the exchange by" date. As Yowp has pointed out repeatedly on his Tralfaz blog even just recently, with original theater program ads as evidence, sometimes a theater was already showing a cartoon weeks in advance of its official (or commonly believed) "release date".

I think its worth pointing out that Leonard Maltin's Of Mice and Magic was published in its second edition back in 1987 and hasn't been republished or updated since. So any error that slipped through the editors fingers back then are still obviously in the book as well as any new information that were not known at the time of publication or shorts & feature films released after the book was published.

While I can understand why the book might not get republished...on the whole I'm surprised that Jerry Beck who was co-author on this book doesn't list corrections to the filmography some where on his CartoonResarch website.