A reexamination of the well-known Hafele-Keating time dilation experiment reveals an overlooked assumption in standard interpretations. The time interval measurements for all clocks began and ended simultaneously in the laboratory, implying that their measured intervals should be identical. This raises questions about the conventional view that these intervals differ in duration due to motion. A deeper analysis revealed a fundamental issue: applying the same standard seconds to all intervals disregards the fact that moving clocks tick at a slower rate. This implies that the duration of a time unit on a moving clock is longer than a standard second. To resolve this inconsistency, we introduce the concept of variable time units. We further propose modifying the time dilation formula by defining time intervals in terms of discrete clock ticks rather than fixed-duration units, demonstrating that the ratio of time units follows directly from the ratio of clock ticks. However, adopting variable time units leads to the inevitable conclusion that the speed of light must also be variable, contradicting Einstein’s postulate of its invariance. From this perspective, while the Hafele-Keating experiment is widely regarded as empirical confirmation of Einstein’s theory of relativity, a closer analysis suggests an alternative interpretation that questions the assumption of a universally constant speed of light.