An encrypted UTC instant field: Encryptor.Ecto.Binary over the value's ISO 8601 form (ADR-0001 decision 1).

defmodule Payments.Encrypted.DateTime do
  use Encryptor.Ecto.DateTime, vault: Payments.Vault
end

defmodule Payments.Cards.Card do
  use Ecto.Schema

  schema "cards" do
    field :merchant_id, :string
    field :verified_at, Payments.Encrypted.DateTime
  end
end

Everything below the cast/parse pair is Encryptor.Ecto.Binary, called rather than copied: the same closed option set, the same declared "table"/"column" context, the same tenant resolution, the same :binary column, the same exception family, and the vault's bytes stored verbatim. Read that module for all of it; only the differences are documented here.

The cast/parse pair

cast/2 is Ecto.Type.cast(:utc_datetime, value) - Ecto's own caster - and the plaintext is DateTime.to_iso8601/1, parsed back with DateTime.from_iso8601/1.

The textual form is not a choice taken here. cloak_ecto's scalar types write the same one, and ADR-0004's migration hands the migrator a legacy plaintext and re-encrypts it verbatim below the schema layer (ADR-0002 decision 3), so a column arriving from Cloak.Ecto.DateTime is readable through this type only because the two agree about what the bytes say.

A payload that decrypts and then does not parse raises Encryptor.Ecto.SerializationError with {:unparsable, :utc_datetime}: the decrypt succeeded, so it is neither an encryption failure nor an integrity event, which is the row ADR-0001 decision 6 gives that exception. A value that reaches dump/3 without passing cast/2 - through insert_all/3, say

• raises ArgumentError naming the shape it was given and never the value.

UTC only, and sub-second precision is dropped at the cast

:utc_datetime is the primitive this type wraps, so a cast value is always in Etc/UTC and dump/3 refuses a DateTime.t/0 in any other zone rather than storing it. The refusal is the point: the ISO 8601 form of a zoned datetime carries its offset, DateTime.from_iso8601/1 resolves that to the equivalent UTC instant, and accepting one would silently drop the zone on every read instead of on the one write a host can still correct.

Ecto's caster likewise truncates a microsecond field to {0, 0} on the way in, exactly as it does for a plain :utc_datetime column - before any encryption, and visibly in the changeset rather than only on the next read.

Everything else

nil is NULL and is not encrypted (decision 7). The column is :binary, not :utc_datetime - type/1 returns :binary whatever the plaintext was (decision 2). Encrypted columns are not queryable, sortable or uniquely indexable (decision 10), so an "expires before" query or an index-backed ordering over this column is not available and is not quietly approximated here.