In exports, the "unsubscribe at" field exports in international date format. This is perfect - it is intelligible in the US with a mm/dd/yyyy format and internationally with a dd/mm/yyyy format. But "born at" is inconsistent with this: it exports in mm/dd/yyyy format. That gets read by Excel internationally as dd/mm/yyyy format, meaning that someone born on the 3rd of April is interpretted as being born on the 4th of March. Once Excel thinks something is a date, but the wrong date, it is not straightforward to fix. Using international date format for all date fields would resolve this.