When creating an order, you shouldn't be able to say that a passenger is born in the future, as you can't possibly know that! If we try to pass a date in the future to an airline then they'll generally reject it, so we want to validate and return a clear error message to you for further action.
We've added an additional validation for this case to our API, which also takes into account the edge case where the passenger has been born today but in a city that is potentially ahead of your timezone and thus potentially a day ahead (e.g. Sydney, Australia).
We'd love to hear your feedback on this, so please do send us an email or tweet us your thoughts!