How to change a time zone in a data frame?

Question

I am loading a csv into a dataframe using

str <- readLines("Messages.csv", n=-1, skipNul=TRUE)
matches <- str_match(str, pattern = "\\s*([0-9]{2}/[0-9]{2}/[0-9]{4}),\\s*([0-9]{2}:[0-9]{2}:[0-9]{2}),\\s*(Me|Them),\\s*(\\+[0-9]{11,12}),\\s*((?s).*)")
df <- data.frame(matches[, -1], stringsAsFactors=F)
colnames(df) <- c("date","time","sender","phone number","msg")


# Format the date and create a row with the number of characters of the messages
df <- df %>%
mutate(posix.date=parse_date_time(paste0(date,time),"%d%m%y%H%M%S"),tz="Europe/London") %>%           
 mutate(nb.char = nchar(msg)) %>%
 select(posix.date, sender, msg, nb.char) %>%
 arrange(as.numeric(posix.date))

I can change sender names using

# Change the senders' names
df <- df %>%
  mutate(sender = replace(sender, sender == "Me", "Mr. Awesome")) 

But I want to change the time zone for the data from to tz="America/Los_Angeles"

I have tried the follow both without success:

attributes(df)$tz<-"America/Los_Angeles"

this compiles but nothing seems to change

and also this:

df <- df %>%
mutate(date = replace(date, format(date, tz="America/Los_Angeles",usetz=TRUE)))

which gives the error: "Error in eval(expr, envir, enclos) : argument "values" is missing, with no default"

Perhaps I am not specifying the original time zone correctly, but I have no idea really how to check that it went through.

Thanks!

Answer 1

First, you can change the time zone of a POSIXct variable. It is not meaningful to "change the time zone in a data.frame", so setting a "tz" attribute of a data.frame does nothing.

[ Note: it is meaningful, however, to change the time zone of an xts object. See this post. ]

I gather that your timestamps are in GMT and you want to convert that to the equivalent in PST. If this is what you are intending, then this should work:

df$posix.date <- as.POSIXct(as.integer(df$posix.date),
                            origin="1970-01-01", 
                            tz="American/Los_Angeles")

For example:

x <- as.POSIXct("2015-01-01 12:00:00", tz="Europe/London")
x
# [1] "2015-01-01 12:00:00 GMT"
as.POSIXct(as.integer(x),origin="1970-01-01",tz="America/Los_Angeles")
# [1] "2015-01-01 04:00:00 PST"

The issue here is that as.POSIXct(...) works differently depending on the class of the object passed to it. If you pass a character or integer, the time zone is set according to tz=.... If you pass an object that is already POSIXct, the tz=... argument is ignored. So here we convert x to integer so the tz=... argument is respected.

Really convoluted. If there's an easier way I'd love to hear about it.