adjusting-dst-daylight-savings-time.R

#!/usr/bin/env Rscript

#library(plyr)
library(dplyr)
library(lubridate)



  ## Datetimes, 15min increments, that span DST breaks
dt_str <- c(
    # Spring forward
   "2015-03-08 00:00:00"
  , "2015-03-08 00:15:00"
  , "2015-03-08 00:30:00"
  , "2015-03-08 00:45:00"
  , "2015-03-08 01:00:00"
  , "2015-03-08 01:15:00"
  , "2015-03-08 01:30:00"
  , "2015-03-08 01:45:00"
  , "2015-03-08 03:00:00"
  , "2015-03-08 03:15:00"
  , "2015-03-08 03:30:00"
  , "2015-03-08 03:45:00"
  , "2015-03-08 04:00:00"
    # Fall back
  , "2015-11-01 00:00:00"
  , "2015-11-01 00:15:00"
  , "2015-11-01 00:30:00"
  , "2015-11-01 00:45:00"
  , "2015-11-01 01:00:00"
  , "2015-11-01 01:15:00"
  , "2015-11-01 01:30:00"
  , "2015-11-01 01:45:00"
  , "2015-11-01 01:00:00"
  , "2015-11-01 01:15:00"
  , "2015-11-01 01:30:00"
  , "2015-11-01 01:45:00"
  , "2015-11-01 02:00:00"
  , "2015-11-01 02:15:00"
)


  ## Put into data.frame
dat <- data.frame(dt_str) %>%
  ## http://stackoverflow.com/questions/15230446/cumulative-sequence-of-occurrences-of-values 
  ## Add a sequence number, make sure we keep track of row order
  mutate(rownum = sequence(n()))

  ## To convert:
  ## https://cran.r-project.org/web/packages/lubridate/vignettes/lubridate.html
dat <-  dat %>%
    ## Convert to timezone
  mutate(dt_etz = dt_str %>% ymd_hms(tz="America/New_York")) %>%
    ## Calculate whether or not timestamp is thought to be in DST
  mutate(is_dst = dst(dt_etz)) %>%
    ## For fall back, timestamps duplicated.  Keep track of occurrence of timestamp
    ## http://stackoverflow.com/questions/15230446/cumulative-sequence-of-occurrences-of-values 
    ## Added 'rev()' call to ID the first pass in DST 'fall back' time with
    ## different identifier as used when there are not duplicate times.
 group_by(dt_etz) %>%
 mutate(count = n() %>% sequence() %>% rev()) %>%
 ungroup() %>%
  ## Convert to UTC, adjust fall back rows that occurred second
 mutate(dt_utc = ifelse(count != 1, with_tz(dt_etz, "UTC") - hours(1), with_tz(dt_etz, "UTC")) %>% as.POSIXct(origin="1970-01-01", tz="UTC"))

dat %>% print.data.frame()


  ## Check what happens when we cast back UTC time to that repeated hour in fall back DST:
  ## Both of these map to the 1am hour that gets repeated.
ymd_hms("2015-11-01 05:00:00", tz="UTC") %>% with_tz("America/New_York")
ymd_hms("2015-11-01 06:00:00", tz="UTC") %>% with_tz("America/New_York")