title: Converting character dates with R package lubridate fails to parse
body: <p>I try to convert the following character vector into a date class.</p>
  
  <pre><code>&gt; library(lubridate)
  &gt; unemployment_rate$month
  
   [1] "2017-1-1"  "2017-2-1"  "2017-3-1"  "2017-4-1"  "2017-5-1"  "2017-6-1"  "2017-7-1"  "2017-8-1"  "2017-9-1"  "2017-10-1" "2017-11-1" "2017-12-1"
  [13] "2016-1-1"  "2016-2-1"  "2016-3-1"  "2016-4-1"  "2016-5-1"  "2016-6-1"  "2016-7-1"  "2016-8-1"  "2016-9-1"  "2016-10-1" "2016-11-1" "2016-12-1"
  </code></pre>
  
  <p>But when I try to convert the dates with the <code>lubridate</code> package I get the following results and warning message:</p>
  
  <pre><code>&gt; unemployment_rate$month &lt;- ymd(unemployment_rate$month, "%Y-%m-%d")
  
  Error in `$&lt;-.data.frame`(`*tmp*`, "month", value = c(17167, 17198, 17226,  : 
    replacement has 25 rows, data has 24
  In addition: Warning message:
  1 failed to parse.
  </code></pre>
  
  <p>I can't understand why <code>replacement has 25 rows</code> while the data has actualy 24 rows (two years), also which date is so different that <code>1 failed to parse.</code>?</p>
  
  <p>Maybe useful thing to know, I created had originally had to create (concatonated and <code>month.abb</code>) the <code>month</code> vector myself from: two columns with one month column formatted "jan', "feb" etc. and a year column formatted "2016" and "2017".</p>