This application note describes the implementation of the internal RTC (real-time clock) of the 32-bit, ARM Cortex-M3-based medium-density STM32F10xxx microcontroller. It provides a set of continuously running counters, which can be used to implement a clock/calendar function using suitable software. The RTC and backup registers are supplied through a switch that draws power from either the VDD supply or the backup battery. Thus, the RTC runs even when the main supply is switched off. This application note provides an example of how to implement the clock/calendar functionality using the medium-density STM32F10xxx microcontroller in low-power and standard applications. The firmware example also performs leap year counting and tamper time stamping. It provides a low-power mode demonstration and a basic RTC calibration routine to compensate for crystal and temperature variations. The STM3210B-EVAL board supports the firmware example described in this application note without any modification to its hardware.