It doesn't work that easy as you think :) You have three options to show dd-mm:
1. Check how datepicker is implemented in clients/base/fields/date/date.js, and override it in custom folder. You can specify a custom display format in your field definition and your custom code can use it to display the value in existing date field.
3. Or - even simpler - Use a simple text field instead - why even save it as a date field if you don't have a year? You can just save it as a custom varchar(5) field with dd-mm format and parse it as needed. For this, you can just use your of initializing the date picker in record/create view.
