Reconstruct a question, answer pair from this explanation: To answer the question, consider the following: The official ceremony to mark the "turning of the first sod" occurred on 28 July 1923, on the spot at Milsons Point on the north shore where two workshops to assist in building the bridge were to be constructed.
Q:Context: The official ceremony to mark the "turning of the first sod" occurred on 28 July 1923, on the spot at Milsons Point on the north shore where two workshops to assist in building the bridge were to be constructed.

Question: when did construction start on the sydney harbour bridge?
A:28 July 1923