日子 (rì zi) means 'day' as a specific calendar day or a period of time, and also 'daily life' (the quality of one's days). For the 'day' sense, 日子 is used for a particular date or the passage of days (e.g., 好日子 'good day'), while 天 (tiān) is the basic unit for counting days (e.g., 三天 'three days'). For the 'life' sense, 日子 focuses on the everyday experience (e.g., 过日子 'get by'), whereas 生活 (shēng huó) is broader, covering lifestyle and living conditions.