Radio Frequency Identification (RFID) systems are an automatic and contactless technology. However, RFID systems also introduce new privacy and security risks. Many papers have tried to propose various RFID authentication mechanisms to solve the RFID privacy issues. In 2005, Molnar et al. first presented the property of delegation. In 2007, Fouladgar and Afifi proposed a simple delegation scheme for RFID systems. The scheme utilizes a counter to control delegation time, but it cannot do so accurately. In this paper, we aim to use the timestamp to control delegation time and enhance the security of RFID systems.