An Ecto type for a set of days, stored in a date[] column (an array of dates), for the multiple-date picker.

# The migration
add :days, {:array, :date}, default: []

# The schema
field :days, Koyomi.Dates, default: []

SQLite has no arrays: ecto_sqlite3 keeps the list in a text column as a JSON array of ISO dates, which the type reads back the same.

The picker posts the days as a list (days[]), with an empty first entry so that a form with no day selected still posts the field. The type drops the empty entries, removes repeated days and sorts the rest, so ["", "2027-03-05", "2027-03-01", "2027-03-05"] casts to [~D[2027-03-01], ~D[2027-03-05]]. It also casts a list of Date and a comma-separated string.