Birthday is not the same as Date.

A Mailchimp Birthday field is intended for recurring birthdays and uses month/day without a year, such as 07/21. A normal Date field stores a full date.

Birthday: MM/DD

If a column is mapped to Birthday, a full birth date such as 1990-07-21 should be reviewed before import rather than blindly treated as a Birthday value. ReadyRows flags values that do not match MM/DD.

Date fields

Mailchimp Date merge fields support several full-date patterns. ReadyRows recognizes common documented patterns such as YYYY-MM-DD, YYYY/MM/DD, DD-MM-YYYY and MM/DD/YYYY, and flags ambiguous or unsupported values for review.