in the US the key event is when the income becomes available to you, not when you collect it. The proper thing to do in your hypothetical case (where you didn'

in the US the key event is when the income becomes available to you, not when you collect it. The proper thing to do in your hypothetical case (where you didn't see the message until well into the next tax year) would be to file an amended return for the previous year declaring the recently discovered income.