Danyele,
I just want to an app I have. XPages.TV. I added a date field to the notes form. Just a date field. no default formula or anything.
I want into an xpage where I can create documents and added a computed field control and bound it to the date field from the form. I set it to display the Date and Time.
In the postNewDocument event of the data source I added this:
authorDoc.replaceItemValue("dateCreated", @Now());
This gave me date with time and saved the time value to the backend notes document.
I changed the computed field to use date time picker and set it to show both and that seemed to work ok as well.
You are using a date field on the notes forum? are you using the datepicker? something else like the dojo version?
What's different between your use case and mine?
Dave