Every quantum entity has the complementarity of a wave and a particle. According to the theory of decoherence that presents the criterion for the particle state versus the wave state, the state is dependent on the interaction that causes decoherence. When a quantum entity is in a state of coherence, it indicates that the entity is in a state of superposition with the property of a wave, and when it interacts to cause decoherence, the entity has a property of a particle. This paper presents an investigation of the reason for the nature of the decoherence and complementarity. In this paper, it is proposed that decoherence creates time, which is explained in terms of a discontinuity in space–time. According to the theory of quantum gravity, in the microscopic world, it is argued that the time variable cannot be used and time does not exist. In contrast, the time variable is used as one of the physical quantities of the macroscopic world. In other words, time exists in the macroscopic world and is defined as a physical quantity. Then, the criterion for demarcating the microscopic world, with no existence of time, and the macroscopic world, with the existence of time, is a relevant question. In this paper, it is proposed that decoherence serves as the criterion––that is, in the state of coherence without interactions, there is no passage of time and time has no significance, and when decoherence is created by interactions, time is created in the macroscopic world. Based on this concept and the discontinuity of time argued in the theory of quantum gravity, the concept of complementarity is also explained herein.