cryo111 cryo111 - 3 months ago 16
R Question

merge.zoo removes time zone

The result of

merge.zoo
does not have the same time zone as its input.

Consider the following example

library(zoo)
zoo_a=zoo(data.frame(a=1:5),
seq(as.POSIXct("2014-01-01 00:00:01",tz="UTC"),
as.POSIXct("2014-01-01 00:00:05",tz="UTC"),
by=1)
)
zoo_b=zoo(data.frame(a=1:4),
seq(as.POSIXct("2014-01-01 00:00:01",tz="UTC"),
as.POSIXct("2014-01-01 00:00:05",tz="UTC"),
by=1)
)

zoo_merged=merge(zoo_a,zoo_b)
time(zoo_merged)[1]
#2013-12-31 19:00:01 EST
time(zoo_a)[1]
#2014-01-01 00:00:01 UTC
time(zoo_b)[1]
#2014-01-01 00:00:01 UTC


The time zone associated with
zoo_merged
is not really
EST
but

library(lubridate)
tz(time(zoo_merged)[1])
#""


The time zone attribute seems to have been removed and
R
is probably using some sort of default timezone to display the data.

I can fix this with
lubridate
via

time(zoo_merged)=with_tz(time(zoo_merged),tz="UTC")
time(zoo_merged)[1]
#2014-01-01 00:00:01 UTC


Is there any way to fix this properly, i.e. without having to change the timezone afterwards?
I was thinking of changing the code for
merge.zoo
but there's not a single line of comments in the respective code...

Answer

Solution as suggested by G. Grothendieck

library(xts)

merge2=function(x,y) {
  as.zoo(merge(as.xts(x), as.xts(y)))
}

time(merge2(zoo_a,zoo_b))[1]
#[1] "2014-01-01 00:00:01 UTC"

Or as suggested by 42-

merge3=function(x,y) {
  if ((tmp_tzone<-attr(time(x),"tzone"))!=attr(time(y),"tzone")) {
    message("Timezone attributes of x and y are not the same. Using default tz.")
    return(merge(x,y))
  } else {
    tmp=merge(x,y)
    attr(time(tmp),"tzone")=tmp_tzone
    return(tmp)
  }
}

#input with same tzones 
time(merge3(zoo_a,zoo_b))[1]
#[1] "2014-01-01 00:00:01 UTC"

#input with different tzones
zoo_c=zoo(data.frame(a=1:4),
          seq(as.POSIXct("2014-01-01 00:00:01",tz="EDT"),
              as.POSIXct("2014-01-01 00:00:05",tz="EDT"),
              by=1)
)

time(merge3(zoo_a,zoo_c))[1]
#Timezone attributes of x and y are not the same. Using default tz.
#[1] "2014-01-01 01:00:01 CET"