title: How to convert time::Tm to u64 for JWT expiration date
body: <p>I'm using the <a href="http://mikkyang.github.io/rust-jwt/doc/jwt/index.html" rel="nofollow noreferrer">jwt crate</a> and I want to set the expiration date in the <code>Claims</code> struct. The <code>exp</code> field in <a href="http://mikkyang.github.io/rust-jwt/doc/jwt/claims/struct.Registered.html" rel="nofollow noreferrer"><code>Registered</code></a> took a <code>Option&lt;u64&gt;</code>.</p>
  
  <p>I can retrieve the current date and add 1 day to it by doing: </p>
  
  <pre><code>let mut timer = time::now();
  timer = timer + Duration::days(1);
  </code></pre>
  
  <p>but I don't see how to convert this <code>time::Tm</code> to a <code>u64</code>.</p>