I'm sorry that you were having difficulty in adding contact's birthdays within your account using Add Multiple Contacts. We do not have that feature available for import by multiple contacts, Could you please add that feature request here.
I did take a look at your account and I see that you were able to add contacts with birthdays later during the day on the 28th. Are you still having trouble? If so, I would recommend importing your contacts using an excel file or a text file. That way you can keep all the information on one screen and it will import perfectly.
Any questions, let us know!