title: Specify Epoch TimeDate Format when serializing/deserializing with Utf8Json
body: <p>i couldn't find enough informations about howto handle DateTimeOffset and TimeSpan during serializing and deserializing with utf8json. i could find Infos about the IJsonFormatter, but not howto implement it correctly..</p>
  
  <p>My example Class looks like this:</p>
  
  <pre><code>public MyClass()
  {
   // This should be converted during serializing/deserializing  to Unix TimeStamp in Milliseconds  
    [JsonFormatter(typeof[CustomDateTimeConverter())]  
    public DateTimeOffSet? BeginDate { get; set; }
  
    // This should be converted to TotalMilliseconds
    public TimeSpan? Elapsed { get; set; } 
  }
  </code></pre>
  
  <p>I have a function to convert the Datetime to EpochTimeStamp from this post: <a href="https://stackoverflow.com/questions/17632584/how-to-get-the-unix-timestamp-in-c-sharp/17632585#17632585">How to get the unix timestamp in C#</a></p>
  
  <p>I tried to add [JsonFormatter(typeof[CustomDateTimeConverter())] to the datamember BeginDate, but need to implement it correctly..</p>
  
  <pre><code>public class CustomDateTimeConverter : IJsonFormatter
  {
      public CustomDateTimeConverter()
      {
         // convert DateTime to Unix Milliseconds for Serialzing
      }
  }
  </code></pre>
  
  <p>Or i am on the wrong way ? How can i achieve this ?</p>
  
  <p>Thank you, for helping !</p>