Thanks for your fast answer, @dragan
This may be a bad decision (newbie's fate), but after having a look at the links you gave and taking good note of @ryan's comments on date creation/etc, I think I will go for the direct manipulation of the database. My reasons are:
1. In several aspects I want the new site to be an straightforward mirror of the old site (dates, visits, urls, etc.).
2. Creating new fields for dates goes against my opinion on what machines should be and should do. Meaning, if the final user is forced to do the machine's work and to understand the machine's entrails in order to use it, having to put there every single detail the machine should be aware of right from the beginning, we'd be better off. The user should be able to focus in only one thing: conceiving and organizing contents. For my experience, if I complicate the editing page with a plethora of fields, the users will run away and in the end I will find myself forced to edit for them, in order to keep the site growing.
Now, let me take a deep breath and get the courage to make some silly questions about the proceeding:
I suppose I need to put the sql query somewhere, to run once, but... Where? Is it in the template.php? Should I write a codePiece.php and put an include_once call in the template.php? (and than get rid of it). And if so, does the codePiece.php need some kind of flag or does php take good care of running only once? Please be indulgent, it is the first time I'm dealing with php, and some parts of the php manual are good for those already with some enlightenment, not for total newbies.
Anyway, I'll have to find the (long) time to experiment with all this, it won't be for today. As soon as I can do it, I'll tell you about the results.
Thanks again for your patience and goodwill.