Mental time is altered by a number of factors and the underlying neural processing involved is highly complicated. Recent research suggests that mental time in patients with particular neurological diseases is perceptually shorter than in normal individuals. This review introduces mental time dysfunction and a model for processing of mental time in Parkinson's and Alzheimer's disease. Although the two diseases show the same dysfunction of mental time in behavior, we expect the underlying neural mechanism to vary in each disease. It is possible that the dysfunction of mental time in Parkinson's disease is caused by the abnormal striatum acting as a pacemaker, while that in Alzheimer's disease is caused by abnormal hippocampal memory.