1) most, if not all, movies and ovas screw horribly with manga canon, especially character dynamics.
2) Gosho has never officially stated the date of Shinichi's birthday in manga. It has only been mentioned in the movies which are not canon.
The movies can never be canon nor the ovas. The anime originals could possibly fit with movies, but those too are not canon. So long as the material was not written by Gosho, it is not canon, and rightly so.
Edit: And ONE MORE THING! Gosho has nothing at all to do with the movies, except doing ONE or TWO key frames per movie and the poster. He has nothing to do with the story what so ever except for the fact that he owns the series and the characters. TMS is licensing the series for the purpose of making the anime and movies. The two are completely separate.