discourse-custom-wizard/assets/javascripts/discourse/components/custom-wizard-field-date.js

import Component from "@ember/component";
import { observes } from "discourse-common/utils/decorators";

export default Component.extend({
  @observes("date")
  setValue() {
    this.set("field.value", this.date.format(this.field.format));
  },

  actions: {
    onChange(value) {
      this.set("date", moment(value));
    },
  },
});