The Enhanced Observed Time Difference (E-OTD) method is one of the emergency positioning technologies that has been standardized for GSM systems in the U.S. In this paper, we apply the E-OTD approach to a (30 kHz) IS-I36 TDMA system in order to study the effect of BTS clock drift on OTD estimation accuracy. We show that the presence of a constant BTS clock drift systematically biases the OTD estimates and causes the RMS OTD error to grow with increasing observation time.