title: How should I deal with 'from' must be of length 1 error?
body: <p>I tried to count days between specific dates.
  I have 2 columns with all character vectors.</p>
  
  <pre><code>start.date &lt;- c("2015-01-10","2015-01-11","2015-02-24")
  end.date &lt;- c("2015-03-10","2015-04-01","2015-06-13")
  date10 &lt;- data.frame(cbind(start.date,end.date))
  date10$start.date &lt;- as.character(date10$start.date)
  date10$end.date &lt;- as.character(date10$end.date)
  str(date10)
  </code></pre>
  
  <p>and the specific dates are from 2015-04-11 to 2015-07-10.
  so I made all date which is between the specific dates by using seq().
      <code>sp.da1&lt;-ymd("2015-04-11")</code>
      <code>sp.da2&lt;-ymd("2015-07-10")</code>
      <code>inteval.da&lt;-seq(sp.da1, sp.da2, by = 'day')</code>
  I wanted to know how many days are between the specific dates.
  I tried to use seq(start.date,end.date,by = 'day') like above, but I get this error: 'from' must be of length 1</p>
  
  <p>Please Help me!!!</p>