An example electronics module for a downhole tool includes a power source that provides an operating voltage and a processor communicably coupled to the power source to receive the operating voltage. A timer is communicably coupled to the power source and the processor and includes a real-time clock, a diode, and a capacitor. The real-time clock is powered by a primary power supply provided by the operating voltage and a backup power supply provided by the capacitor as charged through the diode. The real-time clock is powered by the primary power supply during normal operation and powered by the backup power supply when the primary power supply fails.
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