The New Thermodynamic Understanding of Clocks | Quanta Magazine

The laws which govern how much energy is available are called the laws of thermodynamics and involve a concept called entropy for irreversible thermodynamic processes.

Putting it another way: There is a quantity, usually written as t, which appears in our fundamental description of how change takes place in the physical world. It is also what people are talking about when they ask, “What time is it?” That is what time is. Time is what clocks measure, and everything that changes is a clock.