When a contact is associated with a birthday in ios contacts app, I see that person's birthday appear as an all-day event in the calendar app. However, I don't see how to update the event so that a custom reminder may be set. For example, I would like to be reminded 2 days in advance. I am using ios 6.0.1.
Settings > Mail, contacts, calendars > Default alert times > birthdays
Apparently there are a few options here. You cannot specifically set a custom time at the time of my writing this, but it is possible to pick "a day before", "2 days before", "1 week before" or on day of event. I don't see a way to select a different time though other than 9 am.