title: What are valid input DATE formats for the (Linux) date command?
body: <p>The following command generates the date for the next day:</p>
  
  <pre><code>date -d "20150615 12:00 +1 day" +%Y%m%d
  20150616
  </code></pre>
  
  <p>I would like to specify my own INPUT date format, such as:</p>
  
  <p>2015_06_15</p>
  
  <p>But the date command does not like this format and complains about invalid date:</p>
  
  <pre><code>date: invalid date '2015_06_15 12:00 +1 day'
  </code></pre>
  
  <p>Is it possible to use such a date format? And if so how could I do this.</p>