title: chrono::format::strftime doesn't work with UNIX timestamp
body: <pre><code>use chrono::format::{parse, Item, Parsed};
  
  static SUPPORTED_DATETIME_FORMATS: &amp;[&amp;'static str] = &amp;[&quot;%s&quot;,];
  
  static ref PARSED_DATETIME_FORMATS : Vec&lt;Vec&lt;Item&lt;'static&gt;&gt;&gt; = SUPPORTED_DATETIME_FORMATS
      .iter()
      .map(|format| StrftimeItems::new(format).collect())
      .collect();
      
  fn parse(s: &amp;str) -&gt; Option&lt;i64&gt; {
      for format in PARSED_DATETIME_FORMATS.iter() {
          let mut parsed = Parsed::new();
          let dt = parse(&amp;mut parsed, &amp;s, format.iter().cloned()).and_then(|_| {
              DateTimeParser::apply_default_values(&amp;mut parsed);
              parsed.to_datetime()
          });
          if dt.is_ok() {
              return dt.ok().and_then(|dt| DateTimeParser::extract_ticks(&amp;dt));
          }
      }
      return None;
  }
  </code></pre>
  <p>Unix timestamp should be supported according to the docs:
  <a href="https://docs.rs/chrono/0.3.1/chrono/format/strftime/index.html" rel="nofollow noreferrer">https://docs.rs/chrono/0.3.1/chrono/format/strftime/index.html</a></p>
  <p>However:
  let result = parse(&quot;994518299&quot;);
  // result = None</p>
  <p>Anyone knows why?</p>