Complex sentence: Bolivar returned to Caracas on 12 Jan.1827, but returned to Bogota on 10 Sept.1827 to assume absolute power, setting the date of the constituent congress, 2 Jan.1830, as the day he would surrender power.
Split into simpler sentences: Bolivar returned to Caracas on 12 January 1827, and then back to Bogotá on 10 September 1827 to assume absolute power. He set the date of the constituent congress, 2 January 1830, as the day he would surrender power.