This study explores the impact of serious illnesses, such as cancer, on patients' time preferences in medical decision-making. Specifically, we assess how patients value extending their lifespan by one year under varying survival prognoses through three experimental studies. The findings reveal that patients exhibit a higher Subjective Discount Rates (SDR) in their medical decisions after a serious illness diagnosis. Notably, this difference in individual health also affects the time preferences of their family members. Additionally, the subjective contextual setting of the illness can also increase an individual's SDR levels. The research highlights a tendency for patients and families facing a potential short life expectancy to focus more on immediate concerns, leading to potentially shortsighted and irrational medical choices. This behavior often results in regret during the end-of-life stage. These insights are vital for healthcare professionals in optimizing treatment plans and for policymakers in understanding patient behaviors more comprehensively. The study emphasizes the need for considering psychological and behavioral changes in patients grappling with severe health challenges.