title: Narrow a Joda-Time partial
body: <p>What's the best way to construct a partial from another partial which conains all the necessary fields (e.g. <code>YearMonth</code> from <code>LocalDate</code>? One way I can see is to convert to a full instant and back, that is</p>
  
  <pre><code>YearMonth ld2ym(LocalDate ld) {
      return new YearMonth(ld.toDateTime(DateTime.now()));
  }
  </code></pre>
  
  <p>but it seems like a more efficient way should be possible.</p>