get_tz_list.R

get_tz_list <- function() {
  big_list <- lapply(OlsonNames(), function(tz) {
    dates <- as.POSIXlt(seq(as.POSIXlt("2000-01-01 12:00:00", tz = tz), 
                 as.POSIXlt("2000-12-31 12:00:00", tz = tz), 
                 by = "1 day"))
    gmt_offset <- dates$gmtoff
    tz_code <- dates$zone
    is_dst <- c(NA, FALSE, TRUE)[dates$isdst + 2]
    gmt_offset[is.null(gmt_offset)] <- 0L
    tz_code[is.null(tz_code)] <- NA_character_
    # Possibly get start and end dates of DST?
    unique(data.frame(tz, 
               tz_code,
               is_dst = is_dst,
               gmt_offset_h = gmt_offset / 3600, 
               stringsAsFactors = FALSE))
  })
  
  do.call("rbind", big_list)
}

get_tz_list_more.R

olson_names <- OlsonNames()

get_tz_list <- function() {
  yr <- format(Sys.Date(), "%Y")
  big_list <- lapply(olson_names, function(tz) {
    dates <- as.POSIXlt(seq(as.POSIXct(paste0(yr,"-01-01"), tz = tz), 
                            as.POSIXct(paste0(yr,"-12-31"), tz = tz), 
                            by = "1 day"))
    offs <- get_tz_offset(tz, as.character(dates))
    unique(offs[, setdiff(names(offs), "date")])
  })
  
  do.call("rbind", big_list)
}

get_tz_offset <- function(tz, dates) {
  if (!tz %in% olson_names) {
    stop("tz is not a valid timezone. See ?OlsonNames")
  }
  if (!is.character(dates)) {
    stop("'dates' must be a charcter vector of dates in YYYY-MM-DD format")
  }
  dates <- as.POSIXlt(as.POSIXct(dates, tz = tz))
  gmt_offset <- dates$gmtoff
  zone <- dates$zone
  is_dst <- c(NA, FALSE, TRUE)[dates$isdst + 2]
  gmt_offset[is.null(gmt_offset)] <- 0L
  zone[is.null(zone)] <- NA_character_

  data.frame(tz_name = tz, 
             date = dates,
             zone,
             is_dst = is_dst,
             gmt_offset_h = gmt_offset / 3600, 
             stringsAsFactors = FALSE)
}

get_tz_list()