Example systems and methods are provided for maintaining a system clock during a sleep mode of a mobile communication device. An example system may include a high frequency clock circuit configured to generate a system clock, and a low frequency clock circuit configured to generate a sleep clock, where the sleep clock has a lower frequency than the system clock. The example system may further include a sleep system configured to deactivate the system clock in response to the mobile communication device entering sleep mode. The sleep system may include a sleep counter configured to use the sleep clock to maintain a sleep count for the deactivated system clock during the sleep mode, and a calibration unit configured to activate a calibration clock at periodic intervals during the sleep mode and utilize the calibration clock to calibrate a frequency of the sleep clock.