title: Calculating a total number of unique days of data
body: <p>I am working on a large dataset showing how people travel.  I need to calculate the amount of unique days people travel on.  The table below shows ID, which is unique to each particular person.  Associated with each ID is the dates they have travelled on - for some people this may be one trip per day, for others there may well be multiple trips on each day (e.g. person "1" took two trips on the 4th).  What I need R to do is pick out the total number of unique days for all people in the dataset (e.g. person 1 = 2, person 2=3, person 3=1, person 4=2 - therefore the total using the mini-dataset below should be 8.</p>
  
  <pre><code>ID = c(1,1,1,2,2,2,2,3,4,4,4,4)
  date = c("4th Nov","4th Nov","5th Nov","5th Nov","6th Nov","7th Nov","7th Nov","8th Nov","6th Nov","6th Nov","7th Nov","7th Nov")
  data&lt;-data.frame(ID,date)
  </code></pre>
  
  <p>Any suggestions on the coding for R would be gratefully received.</p>
  
  <p>Many thanks.</p>