A birthdate is composed of a day, a month and a year. Well, in an ideal world, because in practice, most of the time you don't know the year of birth. If it's the birthday of a dear friend of yours who refuses to give you her age, you will only know the day and month to celebrate her birthday every year.
This is why we have added the ability to set a date without knowing the year.
This change applies to birthdates, first met dates and deceased dates. Reminders for those dates will work the same way as before.