There’s no question with renunciation (Immigration and Nationalities Act, s. 349(a)(5)). You are relinquishing your citizenship and notifying the US government of it at the same time, and that’s the date your US citizenship ends.
But what if you relinquished your citizenship by a different method of INS, s. 349(a), such as taking citizenship in another country with the intent to relinquish your US citizenship (349(a)(1))?
The State Department is clear. No matter when you notify the US govt of your relinquishment, once your CLN application is approved, your US citizenship ended on the date you actually relinquished it.
The IRS, however, according to s. 877A of the US Tax Code, considers the date of your relinquishment for IRS purposes is not the date of your actual relinquishment but the date you notified the US government of it (your consulate meeting). This was not the case prior to 2004, however [the relevant section was 7701(n) in 2004 and it was replaced by 877A in 2008].
So, what if you relinquished your US citizenship long ago, but only recently learned of US law and policy changes which now necessitate obtaining a Certificate of Loss of Nationality (a document you probably never even heard of before)? What if the current law regarding IRS and citizenship termination did not exist at the time you relinquished? Logic leads one to the conclusion that laws passed after a person ceases to be a citizen are irrelevant. The IRS has never definitely addressed this issue, however.
Tax lawyers Michael J. Miller and Ellen Brody have just published an excellent article on this matter, Expats Live in Fear of the Malevolant Time Machine, in which they point out the legal, as well as common sense, absurdity of a retroactive application position. It’s very clear reading with useful references to legislation and case law as well.