I understand and I have no precedent to cite either. The Constitution does not recognize the resignation of a Supreme Court Justice, but I think such a thing is implicit in every job.
I think an apt analogy would be if I had an employment contract from 7-31-17 to 7-31-19. On 7-1-18 I tell the appropriate person that I am terminating my employment contract effective 7-31-18. He accepts. He trains the new guy or gal and he or she is ready to start on 8-1-18.
At 11:30pm on 7-31-18 I tell the appropriate person that I changed my mind; that I want to stay on for the duration of my contract. I think that any judge would say that I had forfeited or waived my right to the duration of my contract.
Likewise, Kennedy has a “contract” that he can be an Associate Justice of the Supreme Court so long as he maintains “good behavior.” He exchanged his right to hold that position in exchange for no more pay (he made that promise currently) but said that it would be effective 7-31-18.
Where is the difference?
ETA: An anticipatory breach was the term I was looking for.