Calculating UTC midnight during standard and DST

localMidnightUTC.R

#;; Find the local midnight time in UTC for a given timezone

library(tidyverse)
library(lubridate)
library(glue)

# tgtTimezone <- "America/Chicago"
# tgtTimezone <- "America/Sao_Paulo"
# tgtYear <- 2021
# 
# tibble(utc_dt = seq(ymd_hms(glue("{tgtYear}-01-01T00:00:00Z")),
#                     ymd_hms(glue("{tgtYear + 1}-01-01T00:00:00Z")),
#                     by="hour")) %>%
#     mutate(local_dt = with_tz(utc_dt, tgtTimezone),
#            utcLocalMidnightOffset = glue("{as_date(local_dt) + 1}T00:00:00") %>%
#                ymd_hms(tz=tgtTimezone) %>%
#                with_tz(tz="UTC") %>%
#                hour()) %>%
#     filter((utcLocalMidnightOffset != lag(utcLocalMidnightOffset)) | (row_number() == 1))



getMidnightDSTUTC <- function(tgtTimezone, tgtYear=year(now())) {
    tibble(utc_dt = seq(ymd_hms(glue("{tgtYear}-01-01T00:00:00Z")),
                        ymd_hms(glue("{tgtYear + 1}-01-01T00:00:00Z")),
                        by="hour")) %>%
        mutate(local_dt = with_tz(utc_dt, tgtTimezone),
               utcLocalMidnight = glue("{as_date(local_dt) + 1}T00:00:00") %>%
                   ymd_hms(tz=tgtTimezone) %>%
                   with_tz(tz="UTC"),
               utcLocalMidnightOffset = utcLocalMidnight %>%
                   hour()
               ) %>%
        filter((utcLocalMidnightOffset != lag(utcLocalMidnightOffset)) | (row_number() == 1)) %>%
        mutate(tz=tgtTimezone) %>%
        select(tz, everything()) %>%
        select(-utcLocalMidnightOffset)
}

getMidnightDSTUTC("America/Chicago", 2021)
getMidnightDSTUTC("America/Sao_Paulo", 2021)



dsts_df <-
    c( "America/New_York",
       "America/Chicago",
       "America/Los_Angeles",
       "America/Denver",
       "America/Indiana/Indianapolis",
       "America/Detroit",
       "America/Phoenix",
       "America/Kentucky/Louisville",
       "America/Toronto",
       "America/Vancouver",
       "America/Monterrey",
       "Pacific/Honolulu",
       "America/Indiana/Petersburg",
       "America/Indiana/Marengo",
       "America/Sao_Paulo",
       "America/Puerto_Rico",
       "America/Mexico_City",
       "America/Kentucky/Monticello",
       "America/Indiana/Vincennes",
       "America/Boise",
       "Canada/Eastern",
       "Asia/Singapore",
       "Asia/Calcutta",
       "America/Tijuana",
       "America/Panama",
       "America/Edmonton",
       "America/Anchorage") %>%
    map_dfr(~{
        getMidnightDSTUTC(.x, 2021)       
    })

View(dsts_df)

#;; List local midnight start dates for all Olson Timezones
dsts_df <-
    OlsonNames() %>%
    map_dfr(~{
        getMidnightDSTUTC(.x, 2021)       
    })

dsts_df %>%
    select(tz, utcLocalMidnight) %>%
    arrange(tz, utcLocalMidnight) %>%
    group_by(tz) %>%
    mutate(elt = row_number()) %>%
    pivot_wider(names_from = "elt", values_from = "utcLocalMidnight") %>%
    ungroup() %>%
    set_names(c("tz", "jan", "first", "second")) #%>% write_csv(here::here("dat", "tz-change-listing.csv"))