As global warming continues, we expect extreme climate events to occur more frequently and intensively and the extremes to become normal eventually. Such timing of when the climate extremes in the present climate become normal in a future climate has been estimated in previous studies, but these studies used different methods and definitions, making an interpretation and application of the results difficult. Defining the year of beginning of a new normal climate as the timing of unprecedented climate (TUC), this study suggests a new estimation method using a return level of extreme temperatures. The TUC was estimated using CMIP5 climate model simulation data with an application to the wintertime daily maximum and minimum temperatures in Korea. With a 50-year return level obtained from Generalized Extreme Value distribution of the CMIP5 historical experiment data, overall in the models, TUC was estimated to come in the 2070s under RCP 8.5 (a business- as-usual) scenario and in the 2090s under RCP 4.5 (an intermediate level of emissions) scenario. Using this new method, TUC can be estimated for various fields globally or regionally in different seasons with different variables, providing a useful guideline for climate change mitigation and adaption policy and also a timetable for the actions.