title: Remove year from dates in R
body: <p>I have 20 years worth of weather data, but I'm only interested in the patterns per year. I don't care how June 1995 is different from June 2011, for example. Instead, I want to have 20 values for June 1, 20 values for June 2, etc.</p>
  
  <p><strong>My question:</strong> How do I drop the year portion of a date object, keep the month AND day, while also maintaining the sequential properties of dates? My ultimate goal is a long list of repeated mm/dd values corresponding each to the outcome variable. I'll treat the mm/dd like factors, but in the correct order.</p>
  
  <pre><code># Given this:
  as.Date(c("2014-06-01","1993-06-01", "2013-06-03", "1999-01-31"), "%Y-%m-%d")
  # I want to get this:
  "06-01" "06-01" "06-03" "01-31"
  # That will sort like this
  "01-31" "06-01" "06-01" "06-03"
  </code></pre>
  
  <p>Little hacks like using sub() to drop the year and convert the dash to a decimal doesn't work because then the 1st of the month is the same as the 10th of the month. I also tried turning the dates into character strings, removing the year, and then turning it back into a date... that just made everything year 2014.</p>