title: How do I group my date variable into month/year in R?
body: <p>I have a "date" vector, that contains dates in mm/dd/yyyy format:</p>
  
  <pre><code> head(Entered_Date,5)
  [1] 1/5/1998 1/5/1998 1/5/1998 1/5/1998 1/5/1998
  </code></pre>
  
  <p>I am trying to plot a frequency variable against the date, but I want to group the dates that it is by month or year. As it is now, there is a frequency per day, but I want to plot the frequency by month or year. So instead of having a frequency of 1 for 1/5/1998, 1 for 1/7/1998, and 3 for 1/8/1998, I would like to display it as 5 for 1/1998. It is a relatively large data set, with dates from 1998 to present, and I would like to find some automated way to accomplish this.</p>
  
  <pre><code>&gt; dput(head(Entered_Date))
  structure(c(260L, 260L, 260L, 260L, 260L, 260L), .Label = c("1/1/1998", 
  "1/1/1999", "1/1/2001", "1/1/2002", "1/10/2000", "1/10/2001", 
  "1/10/2002", "1/10/2003", "1/10/2005", "1/10/2006", "1/10/2007", 
  "1/10/2008", "1/10/2011", "1/10/2012", "1/10/2013", "1/11/1999", 
  "1/11/2000", "1/11/2001", "1/11/2002", "1/11/2005", "1/11/2006", 
  "1/11/2008", "1/11/2010", "1/11/2011", "1/11/2012", "1/11/2013", 
  "1/12/1998", "1/12/1999", "1/12/2001", "1/12/2004", "1/12/2005", ...
  </code></pre>