Validating time from a string is hard, there are so many formats it is difficult for you to determine. I suggest making drop down boxes for each field, this gives them no leeway in how to format things. When it comes to input you have the format, the less leeway you give your users the better. You wouldn't think it, but someone will always come around with some weird way they format it that breaks it. Humans can validate information infinitely better than computers, your users likely do not know this.

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