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Concurrent chemoradiation therapy (CRT) is the standard of care for non–small cell lung cancer. We studied the effects of minor differences in the CRT start dates in a cohort of 11,119 patients and found that minimal differences, as few as 3 days, were associated with worse survival rates. Efforts to mitigate the factors that interfere with the synchronous delivery of CRT are needed.We evaluated trends in administration of concurrent chemoradiation therapy (CRT) and how variations in start dates of chemotherapy and radiotherapy affected overall survival (OS) in patients with non–small cell lung cancer (NSCLC) undergoing a course of definitive CRT.Cases of NSCLC treated with definitive CRT were obtained from the National Cancer Database. A survival analysis was performed with Kaplan-Meier curves and Cox proportional hazards models. Propensity score matching was conducted.On a national level, only 48.6% of patients began concurrent CRT on the same day. In a propensity-matched population, starting CRT within 6 days was associated with improved OS (17.9 months) compared with starting 7 to 13 days apart (16.5 months; P = .04). Starting dual therapy within 6 days of each other was associated with a 7% reduction in the risk of death (hazard ratio, 0.93; P = .05). Furthermore, in a propensity-matched cohort, starting CRT within 3 days was associated with longer survival (18.7 months) compared with 4 to 6 days apart (17.5 months; P = .02). Starting treatment 4 to 6 days apart was associated with an 8% increased risk of death (hazard ratio, 1.08; P = .04).A large proportion (48.6%) of patients with unresectable NSCLC do not initiate CRT on the same day as is considered standard by national guidelines. In this population, nonsimultaneous initiation of CRT was associated with differences in OS. Further efforts to understand the mitigating factors and barriers that interfere with timely delivery of concurrent CRT are needed.