Appointment created date not correct

Hello, I noticed when a new appointment is created (from the web site or in the admin panel) the "created" date correctly represent the date when the appointment was created (the current date).
The problem is: if I edit the appointment in the admin panel (ex: I modify the name) the created date change again to the date the appointment was modified.
In other words ... this is not a "created" date, but a "last modified" date.
I don't need the last modified data info, but the appointment creation date, so I would like to know how to make the "created" date behave like its name.
Thanks for your help
Antonio