Dependent Coverage Start Date Formula - 15 days after Date of Birth

Summary:

We have a requirement that a new born child (14 days and below) can be enrolled but the coverage should start when the child reaches 15 days old.

We tried creating dependent coverage formula and the logic is it will add 15 days to the Bday of the child.

Upon testing, the formula is checking the Bday of the previously enrolled Child not the new born child. And if there's is no previously enrolled child, it will set the coverage date based on the default date in the formula ('1971/01/01) + 15 days.

Need help how will the formula look to the Bday of the new born.