New type of "new custom field"

Hello everybody,

In “new custom field”, I want to add a new type “date” which contains in
addition to a text zone, an associated calendar.

I already use this calendar in the field “Date” in “New Ticket” by modifying
the source code of “/opt/rt3/share/html/Elements/SelectDate”, but I want now
to make it a preset type in “new custom field” so that when I want to add a
new custom field of type Date I will choose the “Date Type”.

Thank you in advance for your help.