Autocomplete Birthdate for contact page responsive page

I need to create an autocomplete rule on the Family and Emergency Contacts page making the date of birth mandatory only if the contact has the "Emergency Contact" option as "No". I was able to create the first part of the rule (Emergency contact = no), however I can't find the "date of birth" field so the rule can check if is null or not, even the field is present on the contacts page. How can I find the Date of Birth field?