I’m currently doing it in an input_helper based off of https://dashbit.co/blog/dynamic-forms-with-phoenix:

  defp input(:datetime_select, form, field, opts) do
    date = if Map.has_key?(form.data, field) && !is_nil(Map.get(form.data, field)), do: NaiveDateTime.to_iso8601(Map.get(form.data, field)), else: ""
    content_tag :div, class: "control", phx_update: "ignore" do
      apply(Phoenix.HTML.Form, :text_input, [form, field, [class: "input #{state_class(form, field)}", phx_hook: "datetimeHook", value: date] ++ opts])
    end
  end

import flatpickr from "flatpickr";

export const datetimeHook = { 
  mounted() {
    flatpickr(this.el, {
      altInput: true,
      altFormat: "Y-m-d h:i K",
      dateFormat: "Z",
      enableTime: true,
      parseDate(dateString, format) {
        var wrongDate = new Date(dateString);
        var localizedDate = new Date(wrongDate.getTime() - wrongDate.getTimezoneOffset() * 60000);
        return localizedDate;
      },
    });
  }
}

Saving it as :utc_datetime.

I call it like:
<%= input f, :published_at, field_modifier: "", label: "Publish Date" %>

Though I forget why I have field_modifier: “”

My man do you know how much heartburn I suffered today trying to fix this god forsaken thing and your solution pulled me from the depths of hades out of the clutches of Mephistopheles himself?

That would probably apply to any datepicking library.

I re-read the docs and missed the fact that one could attach phx-update="ignore" to disable patching on specific DOM nodes. I added this to my input field’s parent and wallah! This obviates the need to manage specific instances.

Of course, if anyone has any better ideas, let me know!

That said, I’m surprised that Liveview patches the entire form when validating. I would have thought Liveview would focus on what was changed, not the entire form. I’m guessing that since Liveview patches elements based on changed assigns, that the round trip of the form_data is the change that causes the DOM patch.. ?? I’m hoping I can find some time to look into that more and maybe open a github issue to gather more info.