This is rather an unfair question, IMHO.
The question sheet stated that it was "estimated to have happened in 1804". But who's to say that it didn't actually happen in 1799?
Anyone who answered "18th" was only four or five years away from the estimate. On the other hand, someone who guessed that it happend in 1890, and answered "19th", would have got the points – even though their guess was 86 years away from the estimate.
Better to ask for the year, and give generous leeway – say 20 or 30 years either way.
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