Date fields are great to use when you, for example, want to send autoresponders to your subscribers on a specific date (like their birthday).
Let's say you want to collect the birth dates from your subscribers. For this, you have to use forms and custom date fields.
To create a custom date field, go to Subscribers and click any list. In the list, go to the Custom fields tab. Add your settings and you're good to go!
You will be able to use your newly created date field for specific autoresponders and when collecting user data with forms.
Quick note: The difference between default fields and the ones you created yourself is that you can delete or edit your own fields, but you can’t delete or edit the default fields.