Hey @Della
Welcome to the community and congrats on your first post! Happy to help with your formatting for dates!
See the example below for information around formatting your date and timestamps in a CSV file or API call. You must format dates as either YYYY-MM-DD or MM/DD/YYYY. Note that dates cannot be formatted in the DD-MM-YYYY format. Also using “ . “ to separate year, month and date will not accurately update in Klaviyo.
One question, how are the birthdates being added to a profile? Through CSV upload? Sign-up form? If you are uploading via CSV file, make sure you are mapping the birthday data as a Date property. If you have already mapped this as anything other than a date (ie Text) , you will need to purge all profile data containing this property and reupload with the correct mapping or create a net new property that is mapped as a date,
Hope this helps!