[R] How can I calculate conditional mean in a large dataset including date data

[Majid Iravani]

Dear R users,

I have a dataframe with two columns: first column is date data (e.g. 
1/1/2000 with character format: daily data from 1/1/1970 till 31/12/2003) 
and second column is temperature value. Now I'd like to calculate mean for 
each month in a year (i.e. May 2001, June 1997) and mean for each month in 
all of years. As the number of days in some months is different from others 
I could not write appreciate command for this. Therefore I would greatly 
appreciate if somebody can help me in this case

Thank you
Majid
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  Majid Iravani
  PhD Student
  Swiss Federal Research Institute WSL
  Research Group of Vegetation Ecology
  Zürcherstrasse 111  CH-8903 Birmensdorf  Switzerland
  Phone: +41-1-739-2693
  Fax: +41-1-739-2215
  Email: Majid.iravani at wsl.ch
http://www.wsl.ch/staff/majid.iravani/