title: Select Time Data
body: <p>I only want to select the data between 2015-1 and 2018-1.</p>
  
  <pre><code>usefuldata1 &lt;-usefuldata[usefuldata$zeit &gt; "2014 - 12",]
  
  usefuldata2 &lt;-usefuldata1[usefuldata1$zeit &lt; "2018-2",]
  </code></pre>
  
  <p>Bigger than 2014 works. However, smaller than 2018 does not.
  I get the same number as before.</p>
  
  <p>My data looks as follows:</p>
  
  <pre><code>x &lt;- sample(letters, size = 30, replace = T)
  y &lt;- paste(sample(2010:2020, size = 30, replace = T),
             sample(1:12, size = 30, replace = T), sep = "-")
  
  df &lt;- data.frame(name = x, date = y)
  </code></pre>