No. It's not harmful. The sell-by date is intended more for the store than the consumer. Some allowance is made for the fact that it will likely be stored for a short time before being eaten. A few days or even a week past the date will be fine. A month or two past the date and it might be a different story.
No, you will be fine. The expiration date is usually just something to tell stores to throw it out if it hasn't been sold yet. You should have another few weeks. Plus, I don't think that croissants need refrigerating, so you should be perfectly fine. Cinnamon has so much sugar in it that it will be awhile before it goes bad.