title: date -d not working on mac
body: <p>I am trying to convert a timestamp to a number of seconds and it is not working as intended on a mac. On linux I can run:</p>
  
  <p><code>date -d "2015-06-11 12:39" +%s</code> and I get <code>1434044340</code> as intended. However, when I run that same command on a mac, I get </p>
  
  <pre><code>usage: date [-jnu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ... 
              [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]
  </code></pre>
  
  <p>what is the corresponding mac command and/or how can I get the original command to work on a mac?</p>