This study develops a new method for estimating the departure time of OD flows. Although departure time choice is very sensitive to changes in the travel environment, it has not been included in transportation planning. There exist some theoretical methods, but most have not been applied to practical studies. In addition, the behavior of travelers regarding departure time choice has not been adequately addressed. To overcome these problems, the proposed model takes two behavioral aspects into account. The first is the maximization of in-home activity time before a departure, and the second is the dynamic user equilibrium in departure time choice. This study investigates the effect of incorporating these two aspects through numerical tests. The results indicate that the developed method has a reasonable capability to forecast the resulting departure pattern change after the modification of network attributes. In addition, the method is simpler than existing approaches such as the schedule delay function; thus, it can be helpful for studying practical problems.