unexpected behaviour converting PST8PDT to Etc/GMT+8

[sebdalgarno]

I ran into this issue while converting a real world discharge data set that was provided in PST8PDT (i.e. two 1:00:00 entries on November 07 2021). I had to manually change the second 1:00:00 entry prior to adjusting tz, which doesn't seem right...

# a date time on the eve of end of daylight savings time
x <- dtt_date_time("2021-11-06 23:00:00", tz = "PST8PDT")

# dttr2 (magically) does this correctly - assigns the first 1:00 to PDT and the second 1:00 to PST
x_dtt <- reduce(map(1:4, function(y) dtt_add_hours(x, y)), c)
x_dtt

# note this is better behaviour than lubridate so far!

# we can then successfully convert to GMT and keep the sequence
dtt_adjust_tz(x_dtt, "ETC/GMT+8")

# but when we set the tz on a vector it assigns both 1:00 to PDT
x <- c("2021-11-06 00:00:00", "2021-11-07 1:00:00", "2021-11-07 1:00:00", "2021-11-07 2:00:00")
x_pst <- dtt_date_time(x, tz = "PST8PDT")
x_pst
# same like this
x_pst <- dtt_date_time(x) |> dtt_set_tz("PST8PDT")
x_pst

# which causes this to fail (i.e. should be a continuous sequence)
dtt_adjust_tz(x_pst, tz = "ETC/GMT+8")

# to resolve we have to add an hour to the second 1:00
x_pst[3] <- dtt_add_hours(x_pst[3], 1)
dtt_adjust_tz(x_pst, tz = "ETC/GMT+8")

[aylapear]

This behaviour does match the intention of the function since a vector of strings is passed there is no way to tell whether the value should be PST or PDT if the values are independent (not a series). This means that the PST or PDT needs to be assigned consistently to string values and it appears that "2021-11-07 1:00:00" is coded with PDT and "2021-11-07 2:00:00" is coded with PST. This is unfortunately a limitation but is unavoidable when converting a string based on how time/timezones/daylight savings works.

This scenario can be common with various types of data and so users need to be aware of this issue.

[aylapear]

Potential work arounds could be

1) a function that can detect when this occurs in a data set so a person can manually make the change and confirm it was done correctly 2) a function that can identify and make the correction