If you write a date in YYYY-MM-DD format, there can sometimes be a date which has no duplicate digits. When will be the next time this happens?
Submit your answer to Gene Wirchenko <email@example.com>. Your answer should be in the form of a proof. That means to show how your answer must be correct. The deadline is Wednesday, November 4, 2015 at noon Pacific Time. I will post the answer shortly after.