Neurons in the prefrontal cortex keeps track of elapsed time (even though time was not explicitly relevant) via sequential firing of neurons.  The overlap of sequences depended on the degree of similarity of the item being held in memory.  The time-keeping showed a Weber-fraction-like decrease in precision as time passed.

Compressed timeline of recent experience in monkey lPFC
Zoran Tiganj, Jason A Cromer, Jefferson E Roy, Earl K Miller, Marc W Howard