The principal goal of rice (Oryza sativaL.) breeding is to increase the yield. In the past, hybrid rice was mainlyindicaintra-subspecies hybrids, but its yield has been difficult to improve. The hybridization between theindicaandjaponicasubspecies has stronger heterosis; the utilization of inter-subspecies heterosis is important for long-term improvement of rice yields. However, the different diurnal flower-opening times (DFOTs) between theindicaandjaponicasubspecies seriously reduce the efficiency of cross-pollination and yield and increase the cost ofindica–japonicahybrid rice seeds, which has become one of the main constraints for the development ofindica–japonicahybrid rice breeding. The DFOT of plants is adapted to their growing environment and is also closely related to species stability and evolution. Herein, we review the structure and physiological basis of rice flower opening, the factors that affect DFOT, and the progress of cloning and characterization of DFOT genes in rice. We also analyze the problems in the study of DFOT and provide corresponding suggestions.