title: Change DateTime format of OWIN .expiration and .issued dates
body: <p>I started with the "Individual User Accounts" authentication option when creating a .NET Web API starter template (as described <a href="http://www.asp.net/web-api/overview/security/individual-accounts-in-web-api" rel="noreferrer">here</a>). The token is being correctly generated, however the ".issued" and ".expires" property are in a non-ISO date format. How do I format them with <a href="https://msdn.microsoft.com/en-us/library/az4se3k1(v=vs.110).aspx#Roundtrip" rel="noreferrer"><code>DateTime.UtcNow.ToString("o")</code></a> so it's ISO 8601 compliant?</p>
  
  <pre><code>{
    "access_token": "xxx",
    "token_type": "bearer",
    "expires_in": 1199,
    "userName": "foo@bar.com",
    "Id": "55ab2c33-6c44-4181-a24f-2b1ce044d981",
    ".issued": "Thu, 13 Aug 2015 23:08:11 GMT",
    ".expires": "Thu, 13 Aug 2015 23:28:11 GMT"
  }
  </code></pre>
  
  <p>The template uses a custom <code>OAuthAuthorizationServerProvider</code> and provides a hook to add additional properties to the outgoing token (the 'Id' and 'userName' are my props), but I don't see any way to change existing properties. </p>
  
  <p>I did notice that in the override of<a href="https://msdn.microsoft.com/en-US/library/microsoft.owin.security.oauth.oauthauthorizationserverprovider.tokenendpoint(v=vs.113).aspx" rel="noreferrer"><code>TokenEndpoint</code></a>, I get a <code>OAuthTokenEndpointContext</code> that has a properties dictionary with the .issued and .expired keys. Trying to change these values, however, has no effect.</p>
  
  <p>Thanks much in advance.</p>