Hi Luca,
Actually the correct format for Italian is the one set when the page loads with a dot separator hh.mm but there is a bug/limitation in the time picker portion of our jquery widget where it uses hh:mm
Server side .NET code populates the textbox with the correct format when the page loads but the date/time picker if used formats it incorrectly with hh:mm and it fails to read the correct format from the data in the textbox. So the server side code is correct but the javascript is not matching what the server side is doing. Unfortunately I don't have any immediate fix for this problem, it is hard to find a good time picker that integrates with jquery ui datepicker.
I have logged this problem in our project tracker and will keep my eyes open for a better time picker widget.
Note also that I do NOT recommend modifying the <globalization section of Web.config, you should leave that alone and force Italian from user.config like this:
<add key="UseCultureOverride" value="true" />
<add key="site1culture" value="it-IT" />
<add key="site1uiculture" value="it-IT" />
as documented in Forcing a Specific Language
Best,
Joe