What is the difference between 准时 and 按时?
准 = accurate; precise
准时 = on time/ on schedule (event or action take place at a precisely scheduled/ appointed time)

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按 = according to; base on
按时 = on schedule/ on time (event or action take place strictly following a scheduled/ at an appointed time)
Also: 按时 (base on hourly schedule/ on schedule timely), 按年(base on an annual schedule ) 按月(base on a monthly schedule ), 按人头 (base on head count)
Example: 按时吃药, 按年交税, 按月付款, 按人头付款 ( Take medicine on schedule hourly/ timely, pay taxes base on an annual schedule , pay a sum base on a monthly schedule, pay a sum base on head count)

Note:
准时 can be adverb or adjective - you can say &quot;he is always 准时&quot; but not &quot;he is always 按时&quot; because 按时 is an adverb