title: Convert string to Date in R: month abbreviated "dd-mmm-YYYY"
body: <p>I have a vector of strings which I need to convert to dates. This only works for some of the dates which are in the format of "dd-mmm-YYYY". </p>
  
  <p>Example: this works: <code>strptime("21-Sep-2017", format = "%d-%b-%Y")</code>
  This does not work and returns <code>NA</code>: <code>strptime("21-Dec-2017", format = "%d-%b-%Y")</code> </p>
  
  <p>What am I doing wrong or not seeing?</p>