Microgrid technology has found applications in various areas including institutional campuses, military bases, and remote islands. Many research efforts focus on the development of the microgrid steady-state energy management system (EMS) that can provide the day-ahead schedule or the multi-interval economic dispatch (ED) for distributed energy resources (DERs). This study advances the state-of-the-art microgrid EMS technique by developing a novel EMS approach to achieve a flexible time frame (FTF) DER schedule combined with a single time interval optimal power flow (OPF)-based ED. The FTF DER schedule enables the microgrid to fully utilize the latest updated renewable and load forecasts, and the OPF-based ED integrates realistic microgrid operating constraints in the real-time control. In addition, a mode switching control function is included to re-dispatch DERs in the microgrid for the planned microgrid switching from grid-connected mode to island mode. The feasibility and effectiveness of the proposed microgrid EMS technology are tested and demonstrated using modified IEEE 34-node and 123-node systems.