title: C# GetExpirationDateString() returning data in short date format creating issue
body: <p>I have to check and ensure a certificate is not expired using the below code:</p>
  
  <pre><code>if (DateTime.Parse(cert.GetExpirationDateString()) &lt;= DateTime.Now)
  {
       _logger.Log(LogLevel.Error, "Chain Certificate is Expired");
       return false;
  } 
  </code></pre>
  
  <p><code>cert</code> is a instance of <a href="https://msdn.microsoft.com/en-us/library/system.security.cryptography.x509certificates.x509certificate(v=vs.110).aspx" rel="noreferrer">X509Certificate</a> class of .net.</p>
  
  <p>Issue that am facing is <a href="https://msdn.microsoft.com/en-us/library/system.security.cryptography.x509certificates.x509certificate.getexpirationdatestring(v=vs.110).aspx" rel="noreferrer">GetExpirationDateString</a> function gives the date of expiry in string format(which depends upon the short date format of the current culture)</p>
  
  <p>If the short date format of my machine is <code>DD-MM-YY</code> it returns expiry date <code>21-12-2030</code> as <code>21-12-30</code>, <code>DateTime.Parse</code> function converts <code>30</code> to <code>1930</code> (if the expiry year is >30). </p>
  
  <p>Is there a way by which I can get the certificate expiry always in <code>dd/mm/yyyy</code> format in order to avoid this issue?</p>