I'm sorry to hear you experienced some difficulty updating the birthday fields for your contacts. When you say the format was not accepted are you referring to the .CSV file itself or just the column you had set up for the contact's birthday? If it is the .CSV file itself I would try resaving it in Excel to .XLS to see if that resolves the issue. I have seen it were certain .CSV files are formatted in such a way that our system does not recognize it. If that still doesn't work I would reach out to us at social_support(at)constantcontact(dot)com with your username, the .CSV you are having trouble with, and a reference to this post and we can investigate further.
That said, if it is just the birthday column that is not accepted I would click here for an article on how the different columns, including birthday, should be formatted.
Nick S. Community & Social Media Support
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