Of course, you can explain that 14C dating is based on assumptions, and only ‘works’ in the thousands, not millions, of years range.
However, this response is completely misleading. No real scientist uses carbon-14 dating to try and show something is millions of years old.
- Any tool will give bad results when misused. Radiocarbon dating has some known limitations. Any measurement that exceeds these limitations will probably be invalid. In particular, radiocarbon dating works to find ages as old as 50,000 years but not much older. Using it to date older items will give bad results. Samples can be contaminated with younger or older carbon, again invalidating the results. Because of excess 12C released into the atmosphere from the Industrial Revolution and excess 14C produced by atmospheric nuclear testing during the 1950s, materials less than 150 years old cannot be dated with radiocarbon (Faure 1998, 294).
In their claims of errors, creationists do not consider misuse of the technique. It is not uncommon for them to misuse radiocarbon dating by attempting to date samples that are millions of years old (for example, Triassic "wood") or that have been treated with organic substances. In such cases, the errors belong to the creationists, not the carbon-14 dating method.
- Radiocarbon dating has been repeatedly tested, demonstrating its accuracy. It is calibrated by tree-ring data, which gives a nearly exact calendar for more than 11,000 years back. It has also been tested on items for which the age is known through historical records, such as parts of the Dead Sea scrolls and some wood from an Egyptian tomb (MNSU n.d.; Watson 2001). Multiple samples from a single object have been dated independently, yielding consistent results. Radiocarbon dating is also concordant with other dating techniques (e.g., Bard et al. 1990).