%Calendar.AmbiguousDateTime{possible_date_times calendar Elixir - calendar

Question

I have been doing date time conversions using Calendar everything was working fine for last 4 months but all of sudden am starting some strange behaviour in this,
what am doing is
1..total_days |> Enum.reduce(start_date, fn _i, acc ->
day_of_week = acc |> Calendar.Date.day_of_week_name
rec_head = get_head_tail(schedule[day_of_week])
rec_head |> Enum.each(fn(x) ->
iterate(x, acc, timezone) |> t_download(interval, t_agent)
end)
acc |> Calendar.DateTime.to_erl |> IO.inspect |> Calendar.DateTime.from_erl!(timezone, {123456, 6}) |> IO.inspect |> Calendar.DateTime.add!(86400)
end)
And the
acc |> Calendar.DateTime.to_erl |> IO.inspect |> Calendar.DateTime.from_erl!(timezone, {123456, 6}) |> IO.inspect |> Calendar.DateTime.add!(86400)
give output as
{{2016, 10, 27}, {1, 0, 0}}
%Calendar.DateTime{abbr: "IST", day: 27, hour: 1, min: 0, month: 10, sec: 0,
std_off: 3600, timezone: "Europe/Dublin", usec: {123456, 6}, utc_off: 0,
year: 2016}
{{2016, 10, 28}, {1, 0, 0}}
%Calendar.DateTime{abbr: "IST", day: 28, hour: 1, min: 0, month: 10, sec: 0,
std_off: 3600, timezone: "Europe/Dublin", usec: {123456, 6}, utc_off: 0,
year: 2016}
{{2016, 10, 29}, {1, 0, 0}}
%Calendar.DateTime{abbr: "IST", day: 29, hour: 1, min: 0, month: 10, sec: 0,
std_off: 3600, timezone: "Europe/Dublin", usec: {123456, 6}, utc_off: 0,
year: 2016}
{{2016, 10, 30}, {1, 0, 0}}
But on the last date and time instead of giving me right output as it is giving above, It is giving this
{:ambiguous, %Calendar.AmbiguousDateTime{possible_date_times: [%Calendar.DateTime{abbr: "GMT", day: 30, hour: 1, min: 0, month: 10, sec: 0, std_off: 0, timezone: "Europe/Dublin", usec: {123456, 6}, utc_off: 0, year: 2016}, %Calendar.DateTime{abbr: "IST", day: 30, hour: 1, min: 0, month: 10, sec: 0, std_off: 3600, timezone: "Europe/Dublin", usec: {123456, 6}, utc_off: 0, year: 2016}]}}
Am totally unknown to that reason as It was working fine but all of sudden its giving me these errors. any help?
UPDATE: am using https://github.com/lau/calendar Calendar.

Answer 1

According to the docs, this happens when the input time you provide is ambiguous:
AmbiguousDateTime provides a struct which represents an ambiguous time
and date in a certain time zone. These structs will be returned from
the DateTime.from_erl/2 function when the provided time is ambiguous.
AmbiguousDateTime contains two DateTime structs. For instance they can
represent both a DST and non-DST time. If clocks are turned back an
hour at 2:00 when going from summer to winter time then the “wall
time” between 1:00 and 2:00 happens twice. One of them is on DST and
one of them is not.
This seems to be your case. The times output are 1 a.m. Europe/Dublin time, and the 1 a.m. occurs twice on October 30th 2016: the second time is when at 2 a.m. you turn the clock back when Daylight Savings Time ends.
It seems like you can use the disamb/2 function to disambiguate the result you're getting.