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)
}