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Hour Distances and Daylight Savings

Source: vignettes/hour.Rmd
hour.Rmd

If using period = "hour", it should work as expected at all times when using a time zone that doesn’t have daylight savings, like UTC or EST. If using a time zone with DST, like America/New_York, some additional explanation is required, especially when every > 1.

Spring Forward Gap

In America/New_York’s time zone, as time was about to reach 1970-04-26 02:00:00, daylight savings kicked in and time shifts forward 1 hour so that the next time is actually 1970-04-26 03:00:00.

before_dst <- as.POSIXct("1970-04-26 01:59:59", tz = "America/New_York")
before_dst
#> [1] "1970-04-26 01:59:59 EST"

before_dst + 1
#> [1] "1970-04-26 03:00:00 EDT"

warp_distance() treats hours 1 and 3 as being side by side, since no hour 2 ever existed. This means that hours (0, 1) and (3, 4) get grouped together in the below example.

x <- as.POSIXct("1970-04-26 00:00:00", tz = "America/New_York") + 3600 * 0:7

data.frame(
  x = x,
  hour = warp_distance(x, "hour", every = 2)
)
#>                     x hour
#> 1 1970-04-26 00:00:00 1380
#> 2 1970-04-26 01:00:00 1380
#> 3 1970-04-26 03:00:00 1381
#> 4 1970-04-26 04:00:00 1381
#> 5 1970-04-26 05:00:00 1382
#> 6 1970-04-26 06:00:00 1382
#> 7 1970-04-26 07:00:00 1383
#> 8 1970-04-26 08:00:00 1383

Because period = "hour" just computes the running number of 2 hour periods from the origin, this pattern carries forward into the next day to have a contiguous stream of values. This can be somewhat confusing, since hours 0 and 1 don’t get grouped together on the 27th.

y <- as.POSIXct("1970-04-26 22:00:00", tz = "America/New_York") + 3600 * 0:5

data.frame(
  y = y,
  hour = warp_distance(y, "hour", every = 2)
)
#>                     y hour
#> 1 1970-04-26 22:00:00 1390
#> 2 1970-04-26 23:00:00 1391
#> 3 1970-04-27 00:00:00 1391
#> 4 1970-04-27 01:00:00 1392
#> 5 1970-04-27 02:00:00 1392
#> 6 1970-04-27 03:00:00 1393

One way that you can sort of get around this is by using lubridate’s force_tz() function to force a UTC time zone with the same clock time as your original date. I’ve mocked up a poor man’s version of that function below.

# Or call `lubridate::force_tz(x, "UTC")`
force_utc <- function(x) {
  x_lt <- as.POSIXlt(x)
  x_lt <- unclass(x_lt)
  
  attributes(x) <- NULL
  
  out <- x + x_lt$gmtoff
  
  as.POSIXct(out, tz = "UTC", origin = "1970-01-01")
}

x_utc <- force_utc(x)
y_utc <- force_utc(y)

x_utc
#> [1] "1970-04-26 00:00:00 UTC" "1970-04-26 01:00:00 UTC"
#> [3] "1970-04-26 03:00:00 UTC" "1970-04-26 04:00:00 UTC"
#> [5] "1970-04-26 05:00:00 UTC" "1970-04-26 06:00:00 UTC"
#> [7] "1970-04-26 07:00:00 UTC" "1970-04-26 08:00:00 UTC"

In UTC, hour 2 exists so groups are created as (0, 1), (2, 3), and so on, even though hour 2 doesn’t actually exist in America/New_York because of the DST gap. This has the affect of limiting the (2, 3) group to a maximum size of 1, since only hour 3 is possible in the data.

data.frame(
  x_utc = x_utc,
  hour = warp_distance(x_utc, "hour", every = 2)
)
#>                 x_utc hour
#> 1 1970-04-26 00:00:00 1380
#> 2 1970-04-26 01:00:00 1380
#> 3 1970-04-26 03:00:00 1381
#> 4 1970-04-26 04:00:00 1382
#> 5 1970-04-26 05:00:00 1382
#> 6 1970-04-26 06:00:00 1383
#> 7 1970-04-26 07:00:00 1383
#> 8 1970-04-26 08:00:00 1384

data.frame(
  y_utc = y_utc,
  hour = warp_distance(y_utc, "hour", every = 2)
)
#>                 y_utc hour
#> 1 1970-04-26 22:00:00 1391
#> 2 1970-04-26 23:00:00 1391
#> 3 1970-04-27 00:00:00 1392
#> 4 1970-04-27 01:00:00 1392
#> 5 1970-04-27 02:00:00 1393
#> 6 1970-04-27 03:00:00 1393

Fall Backwards Overlap

In America/New_York’s time zone, as time was about to reach 1970-10-25 02:00:00, daylight savings kicked in and time shifts backwards 1 hour so that the next time is actually 1970-10-25 01:00:00. This means there are 2 full hours with an hour value of 1 in that day.

before_fallback <- as.POSIXct("1970-10-25 01:00:00", tz = "America/New_York")
before_fallback
#> [1] "1970-10-25 01:00:00 EDT"

# add 1 hour of seconds
before_fallback + 3600
#> [1] "1970-10-25 01:00:00 EST"

Because these are two distinct hours, warp_distance() treats them as such, so in the example below a group of (1 EDT, 1 EST) gets created. Since daylight savings is currently active, we also have the situation described above where hour 0 and hour 1 are not grouped together.

x <- as.POSIXct("1970-10-25 00:00:00", tz = "America/New_York") + 3600 * 0:7
x
#> [1] "1970-10-25 00:00:00 EDT" "1970-10-25 01:00:00 EDT"
#> [3] "1970-10-25 01:00:00 EST" "1970-10-25 02:00:00 EST"
#> [5] "1970-10-25 03:00:00 EST" "1970-10-25 04:00:00 EST"
#> [7] "1970-10-25 05:00:00 EST" "1970-10-25 06:00:00 EST"

data.frame(
  x = x,
  hour = warp_distance(x, "hour", every = 2)
)
#>                     x hour
#> 1 1970-10-25 00:00:00 3563
#> 2 1970-10-25 01:00:00 3564
#> 3 1970-10-25 01:00:00 3564
#> 4 1970-10-25 02:00:00 3565
#> 5 1970-10-25 03:00:00 3565
#> 6 1970-10-25 04:00:00 3566
#> 7 1970-10-25 05:00:00 3566
#> 8 1970-10-25 06:00:00 3567

This fallback adjustment actually realigns hours 0 and 1 in the next day, since the 25th has 25 hours.

y <- as.POSIXct("1970-10-25 22:00:00", tz = "America/New_York") + 3600 * 0:5
y
#> [1] "1970-10-25 22:00:00 EST" "1970-10-25 23:00:00 EST"
#> [3] "1970-10-26 00:00:00 EST" "1970-10-26 01:00:00 EST"
#> [5] "1970-10-26 02:00:00 EST" "1970-10-26 03:00:00 EST"

data.frame(
  y = y,
  hour = warp_distance(y, "hour", every = 2)
)
#>                     y hour
#> 1 1970-10-25 22:00:00 3575
#> 2 1970-10-25 23:00:00 3575
#> 3 1970-10-26 00:00:00 3576
#> 4 1970-10-26 01:00:00 3576
#> 5 1970-10-26 02:00:00 3577
#> 6 1970-10-26 03:00:00 3577

As before, one way to sort of avoid this is to force a UTC time zone.

x_utc <- force_utc(x)
x_utc
#> [1] "1970-10-25 00:00:00 UTC" "1970-10-25 01:00:00 UTC"
#> [3] "1970-10-25 01:00:00 UTC" "1970-10-25 02:00:00 UTC"
#> [5] "1970-10-25 03:00:00 UTC" "1970-10-25 04:00:00 UTC"
#> [7] "1970-10-25 05:00:00 UTC" "1970-10-25 06:00:00 UTC"

The consequences of this are that you have two dates with an hour value of 1. When forced to UTC, these look identical. The groups are as you probably expect with buckets of hours (0, 1), (2, 3), and so on, but now the two dates with hour values of 1 are identical so they fall in the same hour group.

data.frame(
  x_utc = x_utc,
  hour = warp_distance(x_utc, "hour", every = 2)
)
#>                 x_utc hour
#> 1 1970-10-25 00:00:00 3564
#> 2 1970-10-25 01:00:00 3564
#> 3 1970-10-25 01:00:00 3564
#> 4 1970-10-25 02:00:00 3565
#> 5 1970-10-25 03:00:00 3565
#> 6 1970-10-25 04:00:00 3566
#> 7 1970-10-25 05:00:00 3566
#> 8 1970-10-25 06:00:00 3567

Conclusion

While the implementation of period = "hour" is technically correct, I recognize that it isn’t the most intuitive operation. More intuitive would be a period value of "dhour", which would correspond to the “hour of the day”. This would count the number of hour groups from the origin, like "hour" does, but it would reset the every-hour counter every time you enter a new day. However, this has proved to be challenging to code up, but I hope to incorporate this eventually.