The time of day of administration (chronotherapy) of certain medications can affect both their toxicity and efficacy. In this pragmatic, multicenter trial, women starting adjuvant endocrine therapy (ET) for hormone receptor-positive early-stage breast cancer were randomized (1:1) to either morning or evening administration. The primary endpoint was endocrine toxicity/tolerability measured by the change in total Functional Assessment of Cancer Therapy-Endocrine Subscale (FACT-ES) score from baseline to 12-weeks. Secondary endpoints included: endocrine toxicity/tolerability and quality of life (FACT-ES and FACT-B) from baseline to 4, 8, 12, and 52 weeks, non-persistence or non-adherence, and patient preference for timing of ET. Between June 30, 2021, and March 18, 2022, 245 eligible participants were randomized to either morning (122/245, 49.8%) or evening ET (123/245, 50.2%). In the overall population, there was no statistical difference in the change in total FACT-ES score from baseline to 12 weeks (p = 0.086). There were no statistically significant differences for any of the secondary endpoints between the two groups. The study provides evidence for the enthusiasm of patients and investigators to take part in chronotherapy studies. Additional prospective studies should be performed to assess how the timing of ET affects survival outcomes to ensure optimal patient care. Trial Registration: ClinicalTrials.gov, NCT04864405.