Ordering posts by anniversary using only day and month

I defined a custom field named crimedate using standard date format Y-m-d. I defined the filters to make ordering by this date work in the post list of the backend, which works fine.

Now I need the possibility to show posts ordered by anniversary, using only day and month of the crimedate, but I don’t know how to do that:

Is it possible to sort dates only by day and month ignoring the year in hook pre_get_posts? Or do I have to save days and month in two separte fields to make that work?