this here is a rare case we have evidence of what the linguists call theme vs rheme in english. in plain terms, the english sentence is not about when the dude was happy but about how he was doing in the 20th century. the pre-position of the adverbial is significant. to make the czech sentence match the theme/rheme with your word order, one would have to use a weird sentence intonation.

That would be the Second Position Rule for jsem. Ve dvacátém století is the first unit of meaning in this translation of the sentence.