Complex sentence: A long-standing campaign to implement universal suffrage had been ignored by the Chinese central government until December 2007, when 2017 and 2020 were announced as dates when the Chief Executive and Legco, respectively, would be elected via universal suffrage.
Split into simpler sentences: A long-standing campaign to implement universal suffrage had been ignored by the Chinese central government. Year 2017 and 2020 were announced as dates when the Chief Executive and Legco, respectively, could be elected via universal suffrage.