Does Elixir’s DateTime.from_naive/3 support selecting the later offset during ambiguous DST transitions?

Answer

No, DateTime.from_naive/3 does not accept a :fold option. Passing fold: :last raises an ArgumentError for an unknown option. However, the function does support selecting the later offset during ambiguous DST fallback transitions through the :ambiguous option, not :fold.

How it works

The :ambiguous option defaults to :first. Setting ambiguous: :last instructs the function to resolve ambiguous local times to the second occurrence, which corresponds to the later UTC offset after clocks fall back.

It is important to distinguish this from the :fold concept used in some other time APIs. Elixir's standard DateTime module does not expose fold metadata in the return value; it returns only {:ok, datetime} or {:error, reason}. You cannot query which offset was applied after the fact.

Version assumptions

This behavior is consistent with Elixir 1.16 and 1.17. As of those versions, there is no announced plan to add a :fold option or fold reporting to the standard library.

Verification

You can confirm the distinction in IEx using the America/New_York fallback on November 5, 2023, when 01:30 occurs twice:

# Default resolves to the earlier offset
DateTime.from_naive(~N[2023-11-05 01:30:00], "America/New_York")
# => {:ok, ~U[2023-11-05 05:30:00Z]} (UTC-04:00)

# Explicitly select the later offset
DateTime.from_naive(~N[2023-11-05 01:30:00], "America/New_York", ambiguous: :last)
# => {:ok, ~U[2023-11-05 06:30:00Z]} (UTC-05:00)

# :fold is not a recognized option
DateTime.from_naive(~N[2023-11-05 01:30:00], "America/New_York", fold: :last)
# => raises ArgumentError for unknown option :fold

Practical considerations

• Timezone database: Correct offset selection depends on an up-to-date tzdata release. Outdated data can produce wrong offsets during transitions.
• Skipped times: The :ambiguous option only affects ambiguous times. Local times skipped during spring-forward transitions still return :invalid unless you pass :raise or :nil.
• Default risk: Relying on the default :first without explicit handling can silently produce the earlier offset in user-facing applications.

If your application must record which occurrence was chosen, you will need to track that decision in application logic rather than extracting it from the returned DateTime struct.