I don't want to steal anyone's hard work but this is exactly what you're looking for, apparently.
using System.Text.RegularExpressions;
public bool IsValidTime(string thetime)
{
Regex checktime =
new Regex(@"^(20|21|22|23|[01]d|d)(([:][0-5]d){1,2})$");
return checktime.IsMatch(thetime);
}