Check if a timezone is valid in R

Question

I am reading a file that contains timestamps and a timezone specification. I would like to be able to detect if a given timezone on this file is recognized by R or not, and supply my own default in case it isn't.

However, it seems like as.POSIXct silently falls back to UTC if given an invalid timezone, with no error or warning I could catch and handle:

> as.POSIXct("1970-01-01", tz="blah")
[1] "1970-01-01 UTC"

What would be a 'proper' way in R to check if a given timezone is recognized or not?

Answer 1

help("time zones") explains a lot of the issues with time zones in detail and is well worth the read.

Results will vary based on your OS, but example("time zones") shows how you can read a zone.tab file if your OS has one.

tzfile <- "/usr/share/zoneinfo/zone.tab"
tzones <- read.delim(tzfile, row.names = NULL, header = FALSE,
                     col.names = c("country", "coords", "name", "comments"),
                     as.is = TRUE, fill = TRUE, comment.char = "#")
str(tzones$name)
#chr [1:415] "Europe/Andorra" "Asia/Dubai" "Asia/Kabul" "America/Antigua" "America/Anguilla" ...

NROW(tzones)
#[1] 415

head(tzones)
#  country      coords             name comments
#1      AD +4230+00131   Europe/Andorra         
#2      AE +2518+05518       Asia/Dubai         
#3      AF +3431+06912       Asia/Kabul         
#4      AG +1703-06148  America/Antigua         
#5      AI +1812-06304 America/Anguilla         
#6      AL +4120+01950    Europe/Tirane   

Answer 2

You could use a timezone library which has knowledge of time zones. This is from the SVN version of RcppBDT:

R> tz <- new(bdtTz, "America/Chicago")     
R> cat("tz object initialized as:       ", format(tz), "\n")    
tz object initialized as:        America/Chicago   
R>     tzBAD <- new(bdtTz, "blah")     
Error in new_CppObject_xp(fields$.module, fields$.pointer, ...) :    
  Unknown region supplied, no tz object created       
R>  

In general, time zone support is dependent on the operating system. So for a portable solution you need to supply a list of valid time zones from somewhere...

And for what it is worth, I am using the csv file from the Boost sources. A copy of that time zones file is eg here at github.

Answer 3

Just stumbled on this question since I was looking to figure out the same thing. Turned out using the following. Leaving this for anyone who might stumble on this question...

is.valid.timezone <- function(timezone) {
  return(timezone %in% (OlsonNames()))
}

Answer 4

You can also use the Rmetrics package timeDate package to check for timezone.

require(timeDate)

timeDate("1970-01-01", zone = "Africa/Dakar")
## [1] [1970-01-01]

timeDate("1970-01-01", zone = "blah")
## Error in .formatFinCenterNum(unclass(ct), zone, type = "any2gmt") : 
##  'blah' is not a valid FinCenter.