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The “warranty period” of a zero coronary artery calcium (CAC) score—the time until conversion to a detectable score—remains undefined for the Japanese population. This study determined the time to CAC incidence in a cohort of asymptomatic Japanese individuals with a CAC score of 0. This retrospective study analyzed 1,395 asymptomatic Japanese adults (median age 57 years [range 40–90]; 56% men) with a baseline CAC score of 0 who underwent repeated CAC scanning (median follow-up, 4.1 years). The time to conversion to detectable CAC (> 0) was estimated using Kaplan-Meier analysis and Cox proportional hazards models. Warranty periods were defined as the estimated time to reach cumulative incidence thresholds of 10% and 15%. Continuous relationships between baseline characteristics and warranty periods were visualized using spline curves. The progression from a zero to a positive CAC score was significantly associated with sex, age, and baseline risk. The estimated time to reach a 10% cumulative incidence was shorter for men (2.5 years; 95% CI, 2.1–3.0) than for women (4.0 years; 95% CI, 3.0–4.6). Interactions were observed between sex and age (p = 0.006) and sex and atherosclerotic cardiovascular disease (ASCVD) risk (p = 0.001). In men, the warranty period (10% threshold) shortened rapidly with age, decreasing from 3.5 years at age 42 to < 2.0 years at age 68. Conversely, women maintained a period > 3.0 years even at age 68. Higher baseline ASCVD risk scores were associated with shorter warranty periods; for the 10% threshold, the period dropped from 4.1 years at 1% risk to approximately 1.6 years for those with > 20% risk. The warranty period for a zero CAC score in Japanese individuals varies substantially by age, sex, and cardiovascular risk. Older men and high-risk individuals exhibit rapid conversion (< 2 years), suggesting the need for more frequent surveillance.