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Across adult warm-blooded vertebrates, the product of resting heart rate f H and maximum lifespan L is approximately constant: N * = f H L ≈ 10 9 cardiac cycles. This empirical regularity, noted since Rubner (1908), has lacked a widely accepted thermodynamic interpretation. We derive N * ≈ 10 9 from the non-equilibrium second law by treating the adult organism as a metabolic non-equilibrium steady state (NESS) and introducing the empirical closure ė p = σ 0 f, which links entropy production rate to heart rate via a mass-specific parameter σ 0 ∝ M 0 . Under this closure, the lifetime entropy budget Σ = σ 0 N * is approximately species-independent when σ 0 is approximately constant---a condition whose direct calorimetric verification remains the critical outstanding experimental test. We further show that N * is the correct primitive invariant: lifetime energy per unit mass is a derived consequence, valid only when body temperature and the mass-specific entropy cost per cycle are both approximately constant. This framework, which we term the Principle of Biological Time Equivalence (PBTE), is placed on a fully falsifiable footing with explicit assumptions, a domain-of-validity table, and five numerical falsification criteria. We test the framework against a dataset of 230 adult vertebrate species spanning eight taxonomic groups. Ordinary least-squares regression on the n = 43 directly measured non-primate placentals yields slope βˆ = -0.903 ± 0.056 (R 2 = 0.863; F-test p = 0.093 against β = -1). Phylogenetically independent contrasts on 112 endotherm species yield a log 10 f H –log 10 L slope of -0.99 ± 0.04 (p = 0.84 against slope -1), confirming the relation is not a phylogenetic artefact. The WBE kinematic null of zero inter-clade variation is rejected (F = 12.7, p < 0.001). Four warm-blooded clades depart systematically from the mammalian baseline; we derive their longevity deviations from a unified thermodynamic multiplier Φ C = Φ rm duty · Φ rm thermal · Φ rm mito+oxid · Φ rm haz , calibrated to independently measured physiology. For primates, the elevated count ⟨N * ⟩ ≈ (2–3)×10 9 follows from a neuro-metabolic entropy model in which greater neural metabolic investment reduces entropy produced per cardiac cycle. For bats, the extreme longevity (Φ rm bat ≈ 7.9) arises from the multiplicative synergy of cardiac suppression during torpor and an Arrhenius thermal factor during hibernation—two mechanisms acting simultaneously whose thermodynamic motivation has not previously been given. For birds, an adverse thermal penalty (Φ rm thermal = 0.73) and adverse flight duty cycle (Φ rm duty = 0.87) are overcome by mitochondrial coupling efficiency and antioxidant robustness. For cetaceans, extreme diving bradycardia (Φ rm duty = 3.08 for bowhead whales) reveals a near-coincidence trap: the raw heartbeat count N rm obs ≈ N 0 conceals a true thermodynamic budget three times the mammalian baseline. Within this framework, the integral of physiological frequency defines a natural biological proper time, which unifies all longevity mechanisms as Class~1 (time dilation: reduce f) or Class~2 (budget expansion: reduce σ 0 ), generating testable predictions for epigenetic aging clocks. The central outstanding experimental requirement is direct calorimetric verification of σ 0 ∝ M 0 , which would convert PBTE from a statistically supported regularity with thermodynamic motivation into a fully tested conservation law.