1998 has two 9's, as do all the years in the 90's. But when are the last and next years that don't have any repeated digit? Between those dates there is a run of 25 consecutive double-digit years. When is the next time that there will be such a run of double-digit years? The longest possible run of unique-digit years is just 10, for example the years 1-10 or 12-21. Adding any century digits would only force an early match, so 10 is the limit. But what is the longest run of double-digit years since the year 1 A.D. and into the future?
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Marc Kummel / firstname.lastname@example.org