What you’re looking for is validations. I don’t think our built in validations will do datetime comparisons, but you can do it with a custom validation.

validate {MyApp.Validations.InTheFuture, field: :use_by_date} 

defmodule MyApp.Validations.InTheFuture do
  use Ash.Resource.Validation
  
  def validate(changeset, opts) do
    case Ash.Changeset.fetch_argument_or_change(changeset, opts[:field]) do
      :error ->
         # in this case, they aren't changing the field, or providing an argument by the same name
         :ok

      {:ok, value} ->
        if DateTime.after?(DateTime.utc_now(), value) do
          :ok
        else
           {:error, field: opts[:field], message: "must be in the future"}
        end
    end
  end
end

EDIT: The validations guide is here: Validations — ash v3.29.3

So, not sure why the builtin one wouldn’t be working, but you need to be careful with Date.utc_today() like that. That will be referring to the day your app was compiled

Make sure to use &Date.utc_today/0

For the archive and Google: The code in the resource would look like this.

  attributes do
    # [...]

    attribute :use_by_date, :date do
      allow_nil? false
    end
  end

  validations do
    validate {App.Validations.InTheFuture, field: :use_by_date}
  end