#--- Monthly Data TS to XTS ---
  library(xts)

  X  <- ts(1:100, start=c(1976), freq=12)
  
  X3 <- xts(X, order.by=as.Date(time(X)))
  X3["1984"]

  #- XTS to TS 
  X4 <- ts(X3, start=as.numeric(strftime(start(X3), format="%Y")), freq=12)




#--- turn XTS from .CSV file
  D <- read.csv("https://nmimoto.github.io/datasets/SPY.csv", header=T)
  X <- xts(D[,10], order.by=as.Date(D[,"Date"], format="%m/%d/%Y"))



#--- as.xts will have problem this way because of Summer Time ---

library(xts)

X=ts(1:100, frequency=12, start=c(1976))
X2=as.xts(X)
X2["1984"]  #- missing Jan 1984 

as.POSIXct(time(X2))  #- because Jan 1984 counted as Dec 1983

time(X2) <- as.POSIXct(time(X2)) + 6*60*60  #- One way to fix it
X2["1982"]