title: Subtract months from time in format year-month
body: <p>I want to subtract months from given date in year and month format. </p>
  
  <pre><code>global_date = "2017-01"
  </code></pre>
  
  <p>I am converting it with the <code>zoo</code> library as follows:</p>
  
  <pre><code>as.yearmon(global_date) - 0.1
  </code></pre>
  
  <p>but it gives me Nov 2016, I want it as '201612' </p>
  
  <p>How can I do it in R?</p>