hmm. seems the case. I added a bugreport (5584812)
The user can create custom dates in contacts in Addressbook using
applescript, but they cannot delete them
tell application "Address Book"
-- 1 make a custom date to play with
make new custom date at end of custom dates of person 1 with
properties {label:"blind date", value:current date}
save addressbook
--2 check that it is there
tell person 1 to return properties of every custom date
end tell
tell application "Address Book"
--3 THE BUG try and delete it
delete (every custom date whose label is "anniversary") of person 1
--fails
delete custom date 1 of person 1
--also fails
end tell