title: Annual, monthly or daily mean for irregular time series
body: <p>I am a new user of "R", and I couldn't find a good solution to solve it. I got a timeseries in the following format:</p>
  
  <pre><code>&gt;dates  temperature depth   salinity
  &gt;12/03/2012 11:26   9.7533  0.48073 37.607
  &gt;12/03/2012 11:56   9.6673  0.33281 37.662
  &gt;12/03/2012 12:26   9.6673  0.33281 37.672
  </code></pre>
  
  <p>I have an irregular frequency for variable measurements, done every 15 or every 30 minutes depending on the period. I would like to calculate annual, monthly and daily averages for each of my variables, whatever the number of data in a day/month/year is. I read a lot of things about the packages zoo, timeseries, xts, etc. but I can't get a clear vision of what I nead (maybe cause I'm not skilled enough with R...).</p>
  
  <p>I hope my post is clear, don't hesitate to tell me if it's not. </p>