The time based on the rotation of the earth around its own axis is known as Universal time (UT), it is not a inform time scale. Another time scale based on the revolution of earth in its orbit around the sun, called Terrestrial time (TT). The difference between TT and UT is known as delta T (DT). DT varies slowly but rather irregularly. The exact value of DT cannot be predicted because the rate at which earth`s rotation is showing down is not known. Therefore, delta T can only be deducted from observations. The knowledge of the exact value of AT is essential for predicting the correct time of astronomical event or to confirm the time of Historical events. If DT is not considered the result may contain an error of several seconds. The value of DT can be found in almanacs, where it is given in seconds of times at January 1 for each year but for some calculation of the astronomical events we require its values at any date of the year. In such cases an approximate formula can make the life simple. This paper discuss a polynomial approximation to DT for the range of 1620 to 2000 AD published by Jean Meeus and Larry Simons and gives a modified version of this approximation which is much more accurate.