[R] Categorizing by month

[lychang]

Thanks for that Ben, I appreciate it a lot. 

I have another question if you don't mind taking the time to answer it. I
want to create a subset from dataset called "datafile5" with only the
variables "Date," "PM2.5 mean concentration," "SITE_LONGITUDE," and
"SITE_LATITUDE," dating only in September 2009. 

Do you know how I can do this? 



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