Polarization-entangled photons provide intriguing insight into the fundamentals of quantum physics, and are vital technology for large scale quantum computing with linear optics, and extending the distance over which a quantum key can be shared. However, those entangled photon sources that have been implemented so far are typically probed using time-averaged measurements, thus obscuring any quantum correlations that evolve over the lifetime of the state. Here we measure the evolution of quantum correlations over the lifetime of a photon pair state, generated by an optically excited quantum dot at ~10K. Time-dependent oscillations of the entanglement fidelity are observed, and attributed to the spin-dependent phase acquired in the intermediate, non-degenerate, exciton-photon state.