What happened in the middle of the twentieth century that will not happen again for 4,000 years ?
(Hint: See the category).
The only year in the 20th Century that read the same when viewed upside down is 1961.
The next four digit year having this property is 6009.
6009-1961=4048
Therefore, the next occurrence of the four digit year having this property will happen in precisely 4048 years from 1961 c.e. and of course 3987 years from now (2022 c.e.).
Puzzle Solution
