The Christian excuse is that they are using Carbon dating, which is not very accurate.
I don't really know about that, how accurate is carbon dating?
Carbon dating is not used to date dinosaur fossils, as it only works up to around 60,000 years ago. There are other radiometric dating methods that are used for older things, up to a few billion years, however.
Basically, we know the rate at which an element decays and so we can date things based on how decayed that element is inside of something.