Creationists now feel that billions of years worth of radioactive decay has occurred on earth, giving old isotopic dates, but this decay took place in only a few thousand years.
The question is why isotopic dates give such old ages.  In the past Creationists often said it was because the system was disturbed.
There may have been daughter substance present initially or parent substance may have been removed, for example.  However geologists have been careful to account for such possibilities.  Now creationists tend to believe the explanation is different.