title: Milliseconds puzzle when calling strptime in R
body: <pre><code>options(digits.secs = 3);
  
  &gt; strptime("2007-03-30 15:00:00.007", format = "%Y-%m-%d %H:%M:%OS");
  [1] "2007-03-30 15:00:00.007"
  &gt; strptime("2007-03-30 15:00:00.008", format = "%Y-%m-%d %H:%M:%OS");
  [1] "2007-03-30 15:00:00.008"
  &gt; strptime("2007-03-30 15:00:00.009", format = "%Y-%m-%d %H:%M:%OS");
  [1] "2007-03-30 15:00:00.008"
  &gt; strptime("2007-03-30 15:00:00.010", format = "%Y-%m-%d %H:%M:%OS");
  [1] "2007-03-30 15:00:00.01"
  &gt; strptime("2007-03-30 15:00:00.011", format = "%Y-%m-%d %H:%M:%OS");
  [1] "2007-03-30 15:00:00.010"
  &gt; strptime("2007-03-30 15:00:00.999", format = "%Y-%m-%d %H:%M:%OS");
  [1] "2007-03-30 15:00:00.998"
  </code></pre>
  
  <p>I'm confused why there's one millisecond's difference from "009", then again from "011".</p>