If you want to send autoresponders to your subscribers on a specific date like their birthday. Or, lets say, you want to collect birthday dates from your subscribers. You have to use forms and you are going to need a custom data field.
To create a custom data field go to Subscribers and click any list. In the list go to the Custom Fields tab. Add your settings and just like that your new data field is now live. You will be able to use that data field for specific auto responders and when collecting user data using 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 data fields.