How to check if a string value is in a correct time format?

If you didn't want to write methods, you could always check and see if converts successfully. If needed you can use a bool to show whether or not it was valid.

bool passed = false;
string s = String.Empty;
DateTime dt;
try{
     s = input; //Whatever you are getting the time from
     dt = Convert.ToDateTime(s); 
     s = dt.ToString("HH:mm"); //if you want 12 hour time  ToString("hh:mm")
     passed = true;
}
catch(Exception ex)
{

}

You can use TimeSpan.Parse or TimeSpan.TryParse methods for that.

These methods uses this format.

[ws][-]{ d | [d.]hh:mm[:ss[.ff]] }[ws]

Elements between in square brackets ([ and ]) are optional.

TimeSpan.Parse("12:33:25") // Parsing fine
TimeSpan.Parse("03:04:05") // Parsing fine
TimeSpan.Parse("3:4:5") // Parsing fine
TimeSpan.Parse("25:60:60") // Throws overflow exception.

Additional method can be written for the purpose of the string time format validation. TimeSpan structure has got TryParse method which will try to parse a string as TimeSpan and return the outcome of parsing (whether it succeeded or not).

Normal method:

public bool IsValidTimeFormat(string input)
{
    TimeSpan dummyOutput;
    return TimeSpan.TryParse(input, out dummyOutput);
}

Extension method (must be in separate non-generic static class):

public static class DateTimeExtensions
{
    public static bool IsValidTimeFormat(this string input)
    {
        TimeSpan dummyOutput;
        return TimeSpan.TryParse(input, out dummyOutput);
    }
}

Calling the methods for the existing string input; (lets imagine it's initialized with some value).

Normal method:

var isValid = IsValidTimeFormat(input);

Extension method:

var isValid = DateTimeExtensions.IsValidTimeFormat(input);

or

var isValid = input.IsValidTimeFormat();

---

UPDATE: .NET Framework 4.7

Since the release of .NET Framework 4.7, it can be written a little bit cleaner because output parameters can now be declared within a method call. Method calls remain the same as before.

Normal method:

public bool IsValidTimeFormat(string input)
{
    return TimeSpan.TryParse(input, out var dummyOutput);
}

Extension method (must be in separate non-generic static class):

public static class DateTimeExtensions
{
    public static bool IsValidTimeFormat(this string input)
    {
        return TimeSpan.TryParse(input, out var dummyOutput);
    }
}