title: Get monthly means from dataframe of several years of daily temps
body: <p>I have daily temperature values for several years, 1949-2010. I would like to calculate monthly means. Here is an example of the data:</p>
  
  <pre><code>head(tmeasmax)
  TIMESTEP   MEAN.C. MINIMUM.C. MAXIMUM.C. VARIANCE.C.2. STD_DEV.C.      SUM COUNT
  1949-01-01  6.836547       6.65       7.33    0.02850574  0.1688364 1.426652     6
  1949-01-02 10.533371      10.24      10.74    0.06140426  0.2477988 1.426652     6
  1949-01-03 18.746729      18.02      19.78    0.18507860  0.4302076 1.426652     6
  1949-01-04 21.244562      20.09      22.40    0.76106980  0.8723931 1.426652     6
  1949-01-05  3.826716       3.11       5.37    0.52706647  0.7259935 1.426652     6
  1949-01-06  9.127782       8.46      10.26    0.20236358  0.4498484 1.426652     6
  
  str(tmeasmax)
  'data.frame':   22645 obs. of  8 variables:
   $ TIMESTEP     : Date, format: "1949-01-01" "1949-01-02" ...
   $ MEAN.C.      : num  6.84 10.53 18.75 21.24 3.83 ...
   $ MINIMUM.C.   : num  6.65 10.24 18.02 20.09 3.11 ...
   $ MAXIMUM.C.   : num  7.33 10.74 19.78 22.4 5.37 ...
   $ VARIANCE.C.2.: num  0.0285 0.0614 0.1851 0.7611 0.5271 ...
   $ STD_DEV.C.   : num  0.169 0.248 0.43 0.872 0.726 ...
   $ SUM          : num  1.43 1.43 1.43 1.43 1.43 ...
   $ COUNT        : int  6 6 6 6 6 6 6 6 6 6 ...
  </code></pre>
  
  <p><a href="https://stackoverflow.com/questions/14394812/how-to-calculate-monthly-average-from-daily-files-in-r">There is a previous question</a> that I couldn't make heads or tails of. I imagine I can probably use <code>aggregate</code>, but I don't know how to break up the dates into the years and months and then approach the nesting of the months inside the years. I tried a loop inside of a loop, but I can never get nested loops to work.  </p>
  
  <p>EDIT to reply to comments/questions:
  I was looking for the mean of "MEAN.C."</p>