I\'ve been trying to create a Google Spreadsheet plugin from some existing Google App Scripts that I have, and one important part of this app script is Custom Functions.
According to Eric Koleda in [Code.gs - date_add_and_subtract]( https://github.com/google/google-apps-script-samples/blob/master/date_add_and_subtract/Code.gs
) to make available the custom fuctions in an add-on it's required to include at least one add-on menu, but this is not working at this time on testing mode.
NOTE: The originally referred page was removed but the Date add and substract add-on sample code is available at https://github.com/googlesamples/apps-script/tree/master/sheets/dateAddAndSubtract.
The "solution" to test a custom function add-on is to publish the add-on privately so you could avoid the Google review of an untested add-on. Related Q&A: Publish an add-on privately
There are a couple of related GAS issues, please star them: