Thanks to @groony and you

@timoschilling @dlackty I believe this breaks filtering for date fields.
This is a resulting query for range 2016-05-02 - 2016-05-02:

SELECT  1 AS count_column FROM "dates" WHERE ("dates"."starts_on" >= '2016-05-01 23:00:00.000000' AND "dates"."starts_on" < '2016-05-03') LIMIT 30 OFFSET 0

It tries to use date time ranges for date fields resulting in timezone conversion.
Then 2016-05-01 23:00:00.000000 gets downcasted to 2016-05-01 and the results are invalid.

My temporary workaround was to monkeypatch these two methods:

ActiveAdmin::Inputs::Filters::DateRangeInput.class_eval do
  def gt_input_name
    case column.type
    when :date
      "#{method}_gteq"
    else
      "#{method}_gteq_date"
    end
  end

  def lt_input_name
    case column.type
    when :date
      "#{method}_lteq"
    else
      "#{method}_lteq_date"
    end
  end
end

Should I open a separate issue?

@bartoszkopinski yes please