Option 1 - If you really want to create a custom calendar control, then you will probably want to take a look at Kal on GitHub. It is a great looking control that looks just like the antive control in the Calendar app. I contacted the author and he indicated that it doesn't use any private APIs and he says that it has been successfully included in apps that passed App Store review.
Kal on GitHub:
http://github.com/klazuka/Kal
Option 2 - You might want to consider using EventKit, which will allow you to integrate your application with the user's personal calendar. This allows you to add events as if the user were in the Calendar application and doesn't require you to build a separate calendar implementation.
From the EventKit Programming Guide:
The Event Kit and Event Kit UI
frameworks together allow iOS
applications to access event
information from a user’s Calendar
database. You can fetch events based
on a date range or a unique
identifier, receive notifications when
event records change, and allow users
to create and edit events for any of
their calendars. Changes made to
events in a user’s Calendar database
with Event Kit are automatically
synced with the appropriate calendar
(CalDAV, Exchange, and so on). This
document describes Event Kit concepts
and common programming tasks.
Take a look at the programming guide for more details.
http://developer.apple.com/iphone/library/documentation/DataManagement/Conceptual/EventKitProgGuide/Introduction/Introduction.html