Datetime validation in c# issue

Hi, I need to validate the date and time part separately. I am using the following function to determine the correct date and time.

private bool ValidateDateTime(string szDateTime, string szFormat)
        {
            try
            {
                   System.IFormatProvider format = new System.Globalization.CultureInfo("en-US", true);                  
                   return DateTime.TryParseExact(szDateTime, szFormat, format,  
                   System.Globalization.DateTimeStyles.AllowWhiteSpaces, out outDate);                
            }
            catch (Exception ex)
            {
               
            }
            return false;
        }

The inputs and validation results are given below.
1. ValidateDateTime("20081005","yyyyMMdd")  - Pass
2.ValidateDateTime("121212","hhmmss")  - Pass

But if i give the time as 13 like below, it fails.
3.ValidateDateTime("131212","hhmmss")  - Fail

Please let me know how to fix this issue as the time component can be between 0-23.

Regards,
Madhan