This technical paper presents Energetic Time Theory (ETT), which provides both a clear definition of time and a mechanistic explanation for relativistic time dilation—filling two fundamental gaps in modern physics. What is Time? ETT defines proper time as an emergent phenomenon arising from the internal energy dynamics of composite systems. Specifically, time accumulates through internal state changes—the motion of a system's constituents relative to its center of mass. This contrasts with standard physics, which describes how time behaves (dilation, relativity) but offers no definition of what time fundamentally is beyond circular statements like "time is what clocks measure." Why Does Time Dilate? While standard special relativity accurately predicts time dilation through the Lorentz factor γ = 1/√(1 - v²/c²), it provides no physical mechanism. ETT demonstrates that when a system moves relative to an observer, its internal velocities appear reduced due to relativistic transformation (transverse components by γ, parallel by γ²). This reduction in internal motion directly causes slower internal processes—which IS time dilation. The framework derives dτ/dt = 1/γ from first principles using energy partition arguments and makes a distinctive testable prediction: apparent temperature of moving systems should transform as T_apparent = T_rest · (1 + 2γ²)/(3γ⁴), distinguishing ETT from purely geometric interpretations. The document includes complete mathematical derivations using Lagrangian and Hamiltonian formulations, applications to thermal and quantum systems, gravitational time dilation analysis, and worked examples. All standard relativistic predictions are reproduced exactly while providing the physical insight and clear definitions that standard theory lacks.