This paper introduces an energy-efficient method for generating cryptographic session keys in resource-constrained IoT devices using time tokens derived from internal microcontroller timers. The proposed Time-Based Key Generation (TBKG) approach eliminates the need for static keys, stored passwords, or centralized synchronization, making it well-suited for autonomous and intermittently connected systems. A mathematical model is developed to define the acceptable time offset between communicating nodes, ensuring synchronization and cryptographic robustness. The method is implemented in C with real-time operating system support and tested on microcontrollers widely used in low-power embedded systems. Experimental evaluation demonstrates that the proposed solution outperforms traditional key exchange protocols by offering faster execution, lower power consumption, and minimal memory usage. In contrast to conventional approaches such as Diffie–Hellman or pre-shared keys, TBKG enables dynamic session key updates, supports forward secrecy, and reduces the risk of compromise through time-limited token validity and protection against replay attacks. The method’s lightweight architecture makes it applicable to medical, industrial, and sensor-based IoT environments, particularly where battery life, memory capacity, and real-time responsiveness are critical. The findings highlight the method’s potential as a scalable and secure key management solution for modern cyber-physical systems. Future research directions include adapting the method for multi-node networks and integrating it with context-aware cryptographic mechanisms.