We show that the timescale divergence between Universal Time (UT1) and international atomic time (TAI), which is compensated for by the occasional addition of a leap second, is due to the fact that the Système Internationale (SI) second is shorter than the UT second. Celestial mechanicians saw the necessity of introducing a timescale that eliminated the discrepancy between the observed and calculated longitudes of the Moon, Sun, and planets. This timescale, called ephemeris time (ET), was measured and used to calibrate the length of the SI second. It has been shown that ET and TAI are equivalent for all practical purposes. We show that the length of the ET second (and consequently the length of the SI second) was shorter than the length of the UT second at the beginning of the tropical year 1900.0, even though it was intended that the ET second would equal this length. We further show that this difference in the lengths of the UT and SI seconds is due to time dilation. The ET (or equivalently the SI) second is a measure of the scale of coordinate time, while the UT second is a measure of proper time for an observer moving with the Earth. Our calculation of the time dilation effect matches both the difference between the SI and UT seconds and also the leap-second insertion rate to within 0.2% since atomic time began in 1958 up to 2000, when UT was redefined. The deceleration of Earth's rotation contributes less than 1% of this timescale divergence according to the measurements from paleontological records of tidal friction. One possible method to convert from the TAI timescale is to use a multiplicative scalar to obtain a UT timescale. This method would necessitate the insertion of a leap second into the UT timescale only once in approximately 14 decades to account for tidal friction.