将可读时间字符串转换为datetime

I have an alarm clock app, I want when the user enter's "5:00 PM", the system store it as datetime. How do I convert '5:00 pm' to a datetime. If it is already past 5:00 pm and the user enters '5:00 pm', the system should assume the user means tomorrow's '5:00 pm' and return the appropriate datetime.I have an alarm clock app, I want when the user