Are you talking about C-14 dating method? Please read the content from the following website:
http://www.custance.org/Library/Volume2/Part_II/Chapter4.html
I copied one paragraph below, for your convenience:
[SIZE=+1]It may be exclaimed, But you don't really mean that prehistoric man is to be accounted for in this way? What about the time factor? You postulate a few thousand years for all this, whereas we 'know' that man is at least half a million years old. But do we? It is not yet time to say with absolute certainty that radioactive dating methods are completely sound. Are we quite sure that the same atmospheric conditions existed prior to the Flood? It could make all the difference if the answer were no.
Suppose for the sake of argument that there was very little conversion of nitrogen to C-[/SIZE][SIZE=-1]14[/SIZE][SIZE=+1] in the upper atmosphere prior to the Flood, due either to some change in the earth's magnetic field or to a greatly increased percentage of carbon dioxide in the earth's atmosphere, reducing the relative proportion of radioactive carbon dioxide. At present the proportion of carbon dioxide in the atmosphere is about 0.04%, but this might easily have been considerably greater before the Flood. Or suppose the atmosphere had in some way been blanketed against neutron bombardment, then once more the percentage of radioactive carbon dioxide would be greatly reduced. The end result is the same in either case: and we would have the following situation -- an organism dying one year before the Flood might have an extremely small amount of radioactive carbon dioxide. By C-[/SIZE][SIZE=-1]14[/SIZE][SIZE=+1] decay-counting methods, the sample would be estimated to be very, very old, let us say, 30,000 years. On the other hand, an organism dying two years later, that is to say, one year after the atmosphere had been modified somewhat as a side-effect of the Flood, might be found by radiocarbon dating to be only 4,500 years old. Thus the two objects separated in actual fact by only two years, would by C-[/SIZE][SIZE=-1]14[/SIZE][SIZE=+1] dating methods be separated by 25,000 years.
Of course, radiocarbon dating is not the only method used to establish the chronology of prehistory: but tree-ring counting is limited to 2,000 to 3,000 years as a rule, and verve counting, though sometimes considered useful up to 10,000 years, is challenged by some very excellent authorities who would limit its usefulness to little more than half this period. These three are virtually the only "absolute" means of dating the past, and they may well be limited in their validity or feasibility to post-Flood times.[/SIZE]
利用现代的科学方法,对考古样品时间的估算,已经非常准确了。譬如用碳14(<sup>14</sup>C)的衰减法,每 5730 年减少一半,衰减为普通的 <sup>12</sup>C。那么测量样品中剩余 <sup>14</sup>C 的量就可推导出生物死亡的年代,我们就可以将之换算成日历年代。