How to keep date format based on its category?

javascript jquery ruby-on-rails ruby ajax

Issues that I notice:

(minor) You're binding to the click event of the radio button, rather than the change event. Be aware that this won't work for someone using a keyboard to fill out the form.
Whenever the radio button changes, you're showing/hiding all of the day fields on the page.
Reusing values for your id attributes. Those are supposed to be unique on the page. I don't think this is your issue, but it can cause weird bugs. Switch to class.

Here's what I would recommend.

Wrap each "stat" on the form in a div with a class that indicates if it's averaged or instance. i.e. <div class="stat averaged">.
Modify your .erb to output the right class names on the div (so it's correct when the page first loads).
Modify your javascript to change the class name of the parent div ($(this).closest('.stat').removeClass('averaged').addClass('instance');)
Setup css rules to hide the day field for averaged stats (.stat.averaged .day { display: none; })

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