Google Forms include the possibility to perform some sort of validation on user input for checkboxes and text; the functionality is available very easily both via the UI and the API. Unfortunately, it seems that there no support for any sort of validation on date input fields. The API does not include it, and I don't see any callback/trigger that I can use for this purpose.
More specifically, my form contains a field in which the user is requested to input their birthdate, and the form can be submitted only if their age lies in a certain interval (otherwise an error message of some sort should be displayed, possibly directly below the input field like in the other existing validations). All this should go in a form created with the Google Form creation UI.
(Incidentally, in the API documentation for a DateItem I don't even see a function to return the date that the user has chosen, which I would need to implement this.)
https://stackoverflow.com/questions/27626802/google-apps-script-validating-dates-in-form --- the answer is not particularly useful: it shows a simple validation function, but does not describe how to bind it to a google form, which is the nontrivial part.
https://stackoverflow.com/questions/19231531/form-validation-before-submission Not a Google Form, but a Google Apps Script, which is something else entirely (and more difficult to modify at need).
Validate age (>18) Google Forms? Not really date validation, just a workaround.
https://stackoverflow.com/questions/15350114/form-validation-using-client-handler-why-does-input-sequence-order-change-the Again google forms "re-invented" as a Google app. It seems to me that on a Google Form created with the UI I don't have available the object ids that I need to do something similar.