When May and her dad are sitting on the couch talking, the candles behind them change from lit to unlit, the lit again.
They use Carbon dating to establish the age of the sand at 5.5 Billion years. However Carbon dating is only useful up to about 50,000 years (because the relevant Isotope has a half-life of only about 6,000 years.) There's no way they could have established the sand's age with using carbon dating, even assuming that it contains any carbon at all.
Fitz claims to have carbon dated the silicon dioxide sample to 1 billion years older than Earth. In addition to not being able to carbon date anything beyond about 50,000 years, only organic material can be carbon dated. Sand is not organic material.