Abstract We used the times of occultations and eclipses between the components of the 65803 Didymos binary system observed in its light curves from 2003 to 2021 to estimate the orbital parameters of Dimorphos relative to Didymos. We employed a weighted least-squares approach and a modified Keplerian orbit model in order to accommodate the effects from nongravitational forces such as binary YORP that could cause a linear change in mean motion over time. We estimate that the period of the mutual orbit at the epoch 2022 September 26.0 TDB, the day of the DART impact, is 11.921 487 ± 0.000028 hr (1σ) and that the mean motion of the orbit is changing at a rate of (5.0 ± 1.0) × 10⁻¹⁸ rad s⁻² (1σ). The formal 3σ uncertainty in orbital phase of Dimorphos during the planned Double Asteroid Redirection Test (DART) mission is 5.°4. Observations from 2022 July to September, a few months to days prior to the DART impact, should provide modest improvements to the orbital phase uncertainty and reduce it to about 4.°2. These results, generated using a relatively simple model, are consistent with those generated using the more sophisticated model of Scheirich & Pravec, which demonstrates the reliability of our method and adds confidence to these mission-critical results.