When does evening, 晚上， begin in China?
well, you're reading a bad example of written vernacular chinese, trying to pretend it has a classical chinese origin, with invented details.


  天氣晴朗


it's an error. the possible choice of word is 清朗 or 澄朗; both means clear, crisp sky.

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000102154

about the starting time of evening, in ancient; it's 1900 hrs.

one night (一夜) is divided into 5 night-watches (五更); midnight is 三更, 

戌時 (1900 - 2100 hrs) is 一更

子時 (2300 - 0100 hrs) is 三更, that's why we've "三更半夜"

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=Z00000144495