Abstract: People frequently underestimate how long it will take them to complete a task. The view of planning fallacy theory is that during the prediction process, people incorrectly use or ignore their memories of how long similar tasks have taken in the past because they take an overly optimistic outlook. Memory bias theory holds a different explanation: People base predictions of future duration on their memories of how long past events have taken, but these memories are systematic underestimates of past duration. People appear to underestimate event duration because they underestimate past event duration. Although there are some argues between these two theories, from the view of Construal Level Theory, theorists can find the sameness between them, they can coexist

Key words: time underestimation, planning fallacy, memory bias theory, construal level theor