I am reopening this issue because I have not solved it so far.
the mechanism I wrote about, in Platform/Core directory, was correct. In that way I can modify a form field during the save phase.
But, this issue was originally different.
What I mean exactly is that when I click a dateType field in a form, a calendar open (and it’s correct) and the current date appear into the field.
I don’t want the current date to appear into the field.