In the author's analysis of errors made in reproduction, scaling, and estimation of time intervals by human subjects two systems of measurements of time typical of man have been revealed. The first one is supported by the "biological clock" mechanism and measures time by means of dividing it into a continuous series of "immediate presents" which in an average subject equal 0.88 sec. The second system in based on available concepts and speech constructs which are used in the estimation of subjective time. The accuracy of estimations and measurements is a direct function of agreement between the subjective unit of time and the astronomical time unit of I sec. Ability to take into account subjective errors in perception of time is of large importance for efficient execution of a number of applied tasks.