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Timex is a rich, comprehensive Date/Time library for Elixir projects, with full timezone support via the :tzdata package. If you need to manipulate dates, times, datetimes, timestamps, etc., then Timex is for you!
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lib/timezone/timezone_dst.ex
defmodule Timex.Timezone.Dst do
@moduledoc """
Rules for determining if a datetime falls within a daylight savings period.
"""
alias Timex.Date, as: Date
alias Timex.DateTime, as: DateTime
alias Timex.TimezoneInfo, as: TimezoneInfo
@doc """
Check if the provided datetime is in daylight savings time
"""
@spec is_dst?(DateTime.t) :: true | false | :ambiguous_time | :doesnt_exist
def is_dst?(%DateTime{:timezone => %TimezoneInfo{:dst_start_day => :undef}}), do: false
def is_dst?(%DateTime{:year => year, :month => month, :day => day, :hour => hour, :minute => min, :second => sec, :timezone => tz}) do
%TimezoneInfo{
:gmt_offset_dst => dst_shift,
:dst_start_day => dst_start_rule, :dst_start_time => dst_start_time,
:dst_end_day => dst_end_rule, :dst_end_time => dst_end_time
} = tz
dst_start_day = get_dst_day_of_year(dst_start_rule, year)
dst_end_day = get_dst_day_of_year(dst_end_rule, year)
current_day = get_day_of_year({year, month, day})
case is_dst_date(dst_start_day, dst_end_day, current_day) do
:equal_to_start ->
is_dst_start_time(time_to_minutes({hour, min, sec}), time_to_minutes(dst_start_time), dst_shift)
:equal_to_end ->
is_dst_end_time(time_to_minutes({hour, min, sec}), time_to_minutes(dst_end_time), dst_shift)
result ->
cond do
# If DST ends the next day at 00:00, then after 23:00 the day before, we're ambiguous
dst_end_day - 1 === current_day and time_to_minutes(dst_end_time) == 0 ->
cond do
time_to_minutes({hour, min, sec}) >= (23 * 60) -> :ambiguous_time
true -> result
end
# Likewise, if DST started the night before at midnight (24:00), then before 01:00 of the current day, the time doesn't exist
dst_start_day + 1 === current_day and time_to_minutes(dst_start_time) == (24 * 60) ->
cond do
time_to_minutes({hour, min, sec}) < 60 -> :doesnt_exist
true -> result
end
true ->
result
end
end
end
defp is_dst_start_time(current_time, start_time, _shift) when current_time < start_time, do: false
defp is_dst_start_time(current_time, start_time, shift) do
case (start_time + shift) do
# When start time is late, say 2400, it rolls over in to the next day with the shift
shifted when shifted < start_time and current_time < start_time ->
(current_time + shift) >= shifted
# When it doesn't roll over, normal checks apply
shifted when shifted > start_time and current_time >= shifted ->
true
shifted when shifted > start_time and current_time < shifted and current_time >= start_time ->
:doesnt_exist
_ ->
:doesnt_exist
end
end
defp is_dst_end_time(current_time, end_time, shift) do
# Ambigous for the hour before it ends
case (end_time - shift) do
# When the end is at 0:00 and the current is >= 0:00, we're out of DST
shifted when shifted < 0 and current_time >= 0 -> false
# When the end is at any other point in the day, and the current time is between the shifted and end time, ambigous
shifted when current_time >= shifted and current_time < end_time ->
:ambiguous_time
# When the end is at any other point in the day, and we're less than the shifted time, we're in dst
shifted when current_time < shifted ->
true
# When the end is at any other point in the day, and we're greater than the end_time, we're out of dst
_ ->
false
end
end
defp is_dst_date(start_day, _, current_day) when current_day == start_day, do: :equal_to_start
defp is_dst_date(_, end_day, current_day) when current_day == end_day, do: :equal_to_end
defp is_dst_date(start_day, end_day, current_day) when start_day < end_day and (current_day > start_day and current_day < end_day), do: true
defp is_dst_date(start_day, end_day, current_day) when start_day < end_day and (current_day < start_day or current_day > end_day), do: false
defp is_dst_date(start_day, end_day, current_day) when start_day > end_day and (current_day < start_day and current_day > end_day), do: false
defp is_dst_date(start_day, end_day, current_day) when start_day > end_day and (current_day > start_day or current_day < end_day), do: true
defp get_dst_day_of_year({weekday, day, month}, year) when (weekday == :last) or (weekday == 5) do
month_num = Date.month_to_num(month)
day_num = Date.day_to_num(day)
get_last_dst(day_num, month_num, year)
end
defp get_dst_day_of_year({weekday, day, month}, year) when (weekday > 0) and (weekday <= 4) do
month_num = Date.month_to_num(month)
day_num = Date.day_to_num(day)
dst_days = get_day_of_year({year, month_num, 1})
dst_day = :calendar.day_of_the_week({year, month_num, 1})
case (dst_day === day_num) and (weekday === 1) do
true -> dst_days
false ->
adjusted_dst_days = case day_num >= dst_day do
true ->
dst_days + (day_num - dst_day)
false ->
dst_days + (7 - dst_day) + day_num
end
adjusted_dst_days + (weekday - 1) * 7
end
end
defp get_dst_day_of_year(_, _), do: raise(:error, "Invalid weekday")
defp get_last_dst(day_num, month_num, year) do
month_last_days = :calendar.date_to_gregorian_days(year, month_num, 1) + :calendar.last_day_of_the_month(year, month_num)
month_last_date = :calendar.gregorian_days_to_date(month_last_days)
month_last_dayofweek = :calendar.day_of_the_week(month_last_date)
case month_last_dayofweek > day_num do
true ->
month_last_days - (month_last_dayofweek - day_num) - :calendar.date_to_gregorian_days(if year == 0 do 0 else year - 1 end, 12, 31)
false ->
month_last_days - month_last_dayofweek - (7 - day_num) - :calendar.date_to_gregorian_days(if year == 0 do 0 else year - 1 end, 12, 31)
end
end
defp get_day_of_year({year, _, _} = date) do
:calendar.date_to_gregorian_days(date) - :calendar.date_to_gregorian_days(if year == 0 do 0 else year - 1 end, 12, 31)
end
defp time_to_minutes({hours, minutes}), do: (hours * 60) + minutes
defp time_to_minutes({hours, minutes, _}), do: (hours * 60) + minutes
end