Who first worked out this explanation? Was it William Miller? QSEW 15.8
No, this explanation was used before Miller or the Millerite Movement were born. It may have been Johann Petri, a Reformed Church pastor in Germany, who published a tract in 1768 in which he declared that the 2300 days began synchronously with the seventy weeks. (See L. E. Froom, Prophetic Faith of Our Fathers, Volume 2, Page 714.) QSEW 15.9