I ran this question by the vice president of our local branch of a large (American) bank. He said that it’s actually fairly common for a collection company to resell a charged-off debt a few months before the 7 years runs out. The new company then makes an attempt to collect, and reports it to the credit agencies as a new debt, starting the 7-year clock over again. The 7-year period, according to him, is from the last “activity” on the account.
However, some creditors do try this, and Experian *et al * won’t do anything about it unless you dispute the “derog”. Another reason why you have to check your credit report every so often!
DrDeth is dead on. Some collectors DO illegally report incorrect dates in their reporting. They open themselves up to civil litigation by doing so. I reckon all you have to do is prove that their improper reporting of said tradeline stopped you from getting a lower-rate refi on your mortgage, and sue for the difference in interest payments over thirty years.