按时 (àn shí) means 'on time, on schedule' and focuses on completing something by a set deadline or according to a plan. It differs from 准时 (zhǔn shí), which emphasizes punctuality at a specific time (e.g., arriving at 8:00 sharp), and from 及时 (jí shí), which means 'promptly' or 'in time' to avoid a problem. 按时 is an adverb only, used before verbs like 完成 or 起床.