title: Calculating Summary Statistics by Day/Week/Month
body: <p>In R, I have a data-frame of various statistics recorded throughout the day. (For example, heart-rate) . The time-stamps for each measurement-entry are automatically created, and I have already converted them into a POSIXt class element. </p>
  
  <p>The number of observations varies from day to day. </p>
  
  <p>I am wondering how I can calculate summary statistics by day/week/month.</p>