Yours, a friend’s, anyone’s. I’ll find the day and month in 9 questions or fewer.
You only answer yes or no.
Every question splits the remaining candidates in half. Twelve months need at most 4 questions, because 24 = 16 ≥ 12. A month has at most 31 days, which needs at most 5 questions, because 25 = 32 ≥ 31. Together that is 9.
This is binary search with yes-or-no answers instead of ‘higher’ and ‘lower’. The question is always ‘is it in the first half of what is left?’.
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