It is a challenge to use a date field (day, month, year) and to sort only on the day and month. You need this to make a birthday calendar. Sort by date of birth is not possible because then the oldest comes first and you don't have a birthday calendar.
In Drupal 7 you even have a module (birthday). There you fill out some day and month. Disadvantage is that you can not sort on age any more. You don't need an extra module for this... Simply create an extra date field and enter once again the birth dates, but always take the same year (preferably a leap year). So you will be able to sort on birthdays.
I use the professional way .... birthdate being rewritten with a computed field. So the input is only once. With Simple Timeline you can achieve something:
You see, I show the date of birth, without the year (y F). But I sort by another field.
The other field is a computed field of type integer.
It puts the birth date to an integer. Namely the day in the year. I use the date format "z". Thus, January 31 => 31, February 1 => 32, etc. Yes, I also tested the leap years ... February 29 is 59.