I flagged your comment because you were using very inflammatory language without any provocation or reason, precisely as you are doing in this one. Please try to be less vitriolic in your language on the forum. There is no reason to be using terms like “idiot” or saying people would be mentally deficient or whatever if they do things differently than you.

If you want use just :calendar without libraries, you can just do it simpy this way:

defmodule DateHelperTest do
  use ExUnit.Case

  test "date is not older than 3 days" do
    assert DateHelper.not_older_than?({{2018, 1, 9}, {}}, 3)
    assert DateHelper.not_older_than?({{2018, 1, 15}, {}}, 3)
    refute DateHelper.not_older_than?({{2016, 1, 9}, {}}, 3)
    refute DateHelper.not_older_than?({{2018, 1, 7}, {}}, 3)
    refute DateHelper.not_older_than?({{2018, 1, 8}, {}}, 3)
  end
end

defmodule DateHelper do
  def not_older_than?(date, days) do
    diff_days(extract_date(:calendar.universal_time), extract_date(date)) <= days
  end

  defp extract_date({date, _}), do: date

  defp diff_days(d1, d2) do
    :calendar.date_to_gregorian_days(d1) - :calendar.date_to_gregorian_days(d2)
  end
end

# and your code:
if DateHelper.not_older_than?(Ecto.DateTime.to_erl(my_user.signed_up_at), 3) do
 # good to go