[R] Picking out dates in xts

[Lily_stats]

Hi all, 

I like how smart xts and  would like to pick out a period from my xts
object*. I am able to pick out periods of particular years, for example :

conc.xts<-tt.xts['1983']

My question is, how can I pick out say, 
 a) just the first 6 months of 1983. 
b) several years, say 1983 AND 1984
c) 18 month periods ... etc

Your help is really appreciated!

Thanks!




*An ‘xts’ object from 1983-01-11 13:00:00 to 2005-01-02 containing:
  Data: num [1:153086, 1:4] 7 6.2 5.7 4.9 5.7 6.6 6.2 7.4 7 8 ...
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : chr [1:4] "v_a_ref" "DD_a_ref" "v_a_site" "DD_a_site"
  Indexed by objects of class: [POSIXt,POSIXct] TZ: 
  Original class: 'data.frame'  
  xts Attributes:  
 NULL

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