I noticed that this strange behaviour only occurs when I modify a date which also includes a time. If I remove the time from the date, I can succesfully modify the date. I guess that's why I noticed this only on movies I added since version .14, because the imported movies from .11 have no time after the date.
What do mean by "it works properly on those"? Dates should be displayed according to your Windows Regional settings (as set in Control Panel), and this has no bearing on how the dates are stored in the database. When a date is added or changed, it should be handled according to your Regional date setting as well. So if that's "d-m-yyyy," then entering today's date as "16/9" should result in "16-9-2009." Entering "9/16" should be ignored because it's invalid.
Are you saying the date for existing movies is displayed as "d-m-yyyy," but that for new movies is "m-d-yyyy"? If so, something is seriously wrong.
I did a fresh installation of 0.9.9.14, and copied my collection from 0.9.9.11 over. I noticed that when I modify the "date added" on a newly added movie, no matter what I do, it places the month before the day (m-d-yyyy). When I modify movies from my .11 backup, it works properly on those. Anything I can do about this?