We only have month/day data for most people. How can I change the format for the constituent profiles to just do month/day?
When we just have the month/day for the birthdate we set the year to a generic year like 1900. You still are able to get your birthdate notifications and run all the reports you want. It's just that the calculation of the contact's age will be wrong. We use a year that would make the person VERY old so that we know that the year is not known. It has worked for us without any issues.
You only want to store the month and day and not the year of the Birth Date?
In that case you actually do not want to store the Birth Date. That is not a change in format, that is a different kind of information.
My suggestion is to create 2 custom fields for it: an integer field for the month and a second integer field for the day.