fix(datetime-button): render correct text when passing partial date values (#27816)

Issue number: resolves #27797

---------

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issue. -->

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etc). Submit multiple pull requests if needed. -->

## What is the current behavior?
<!-- Please describe the current behavior that you are modifying. -->

Datetime Button passes a parsed value to one of the many text formatting
utilities we have, such as `getMonthAndYear`. However, developers can
pass partial date values such as `2022` or `2022-04` (April 2022).
According to
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date#date_time_string_format,
these are still valid date strings.

However, the `parseDate` utility does not add fallback values. So
passing `2022` will cause the `day` and `month` fields to be
`undefined`. This means that `getNormalizedDate` passes `'//2022'` to
the `Date` constructor.

Some browsers, such as Chrome, will automatically account for the stray
slashes and still return a valid date. Other browsers, such as Safari,
do not do this and will either return "Invalid Date" or throw an error.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Date normalizing utility now has fallback values so we always pass in
a valid date. In the example above, `getNormalizedDate` will now pass
`'1/1/2022'` instead of `'//2022'` to the `Date` constructor.
- Refactored other utils that use `new Date` to make use of
`getNormalizedDate` since they are also impacted.

Note: I added an E2E test instead of a spec test because I want to test
cross-browser behavior to ensure consistency.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!-- If this introduces a breaking change, please describe the impact
and migration path for existing applications below. -->


## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

core/src/components/datetime-button/test/basic/datetime-button.e2e.ts

@@ -122,6 +122,42 @@ configs({ modes: ['md'], directions: ['ltr'] }).forEach(({ title, config }) => {
 
       await expect(dateTarget).toContainText('May 10, 2023');
     });
+    test('should set only month and year when only passing month and year', async ({ page }, testInfo) => {
+      testInfo.annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/27797',
+      });
+
+      await page.setContent(
+        `
+        <ion-datetime-button locale="en-US" datetime="datetime"></ion-datetime-button>
+        <ion-datetime id="datetime" value="2022-01" presentation="month-year"></ion-datetime>
+      `,
+        config
+      );
+      await page.waitForSelector('.datetime-ready');
+
+      await expect(page.locator('#date-button')).toContainText('January 2022');
+      await expect(page.locator('#time-button')).toBeHidden();
+    });
+    test('should set only year when passing only year', async ({ page }, testInfo) => {
+      testInfo.annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/27797',
+      });
+
+      await page.setContent(
+        `
+        <ion-datetime-button locale="en-US" datetime="datetime"></ion-datetime-button>
+        <ion-datetime id="datetime" value="2022" presentation="year"></ion-datetime>
+      `,
+        config
+      );
+      await page.waitForSelector('.datetime-ready');
+
+      await expect(page.locator('#date-button')).toContainText('2022');
+      await expect(page.locator('#time-button')).toBeHidden();
+    });
   });
 
   test.describe(title('datetime-button: locale'), () => {

core/src/components/datetime/utils/format.ts

@@ -113,7 +113,7 @@ export const generateDayAriaLabel = (locale: string, today: boolean, refParts: D
   /**
    * MM/DD/YYYY will return midnight in the user's timezone.
    */
-  const date = new Date(`${refParts.month}/${refParts.day}/${refParts.year} GMT+0000`);
+  const date = getNormalizedDate(refParts);
 
   const labelString = new Intl.DateTimeFormat(locale, {
     weekday: 'long',
@@ -134,7 +134,7 @@ export const generateDayAriaLabel = (locale: string, today: boolean, refParts: D
  * Used for the header in MD mode.
  */
 export const getMonthAndDay = (locale: string, refParts: DatetimeParts) => {
-  const date = new Date(`${refParts.month}/${refParts.day}/${refParts.year} GMT+0000`);
+  const date = getNormalizedDate(refParts);
   return new Intl.DateTimeFormat(locale, { weekday: 'short', month: 'short', day: 'numeric', timeZone: 'UTC' }).format(
     date
   );
@@ -147,7 +147,7 @@ export const getMonthAndDay = (locale: string, refParts: DatetimeParts) => {
  * Example: May 2021
  */
 export const getMonthAndYear = (locale: string, refParts: DatetimeParts) => {
-  const date = new Date(`${refParts.month}/${refParts.day}/${refParts.year} GMT+0000`);
+  const date = getNormalizedDate(refParts);
   return new Intl.DateTimeFormat(locale, { month: 'long', year: 'numeric', timeZone: 'UTC' }).format(date);
 };
 
@@ -183,11 +183,25 @@ export const getYear = (locale: string, refParts: DatetimeParts) => {
   return getLocalizedDateTime(locale, refParts, { year: 'numeric' });
 };
 
-const getNormalizedDate = (refParts: DatetimeParts) => {
+/**
+ * Given reference parts, return a JS Date object
+ * with a normalized time.
+ */
+export const getNormalizedDate = (refParts: DatetimeParts) => {
   const timeString =
     refParts.hour !== undefined && refParts.minute !== undefined ? ` ${refParts.hour}:${refParts.minute}` : '';
 
-  return new Date(`${refParts.month}/${refParts.day}/${refParts.year}${timeString} GMT+0000`);
+  /**
+   * We use / notation here for the date
+   * so we do not need to do extra work and pad values with zeroes.
+   * Values such as YYYY-MM are still valid, so
+   * we add fallback values so we still get
+   * a valid date otherwise we will pass in a string
+   * like "//2023". Some browsers, such as Chrome, will
+   * account for this and still return a valid date. However,
+   * this is not a consistent behavior across all browsers.
+   */
+  return new Date(`${refParts.month ?? 1}/${refParts.day ?? 1}/${refParts.year ?? 2023}${timeString} GMT+0000`);
 };
 
 /**