[8.19] Use fixed dates to test getNextRunAt (elastic#223574) (elastic#224953)

# Backport

This will backport the following commits from `main` to `8.19`:
- [Use fixed dates to test getNextRunAt
(elastic#223574)](elastic#223574)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

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Erdal","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-06-23T19:41:20Z","message":"Use
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BACKPORT-->

Co-authored-by: Ersin Erdal <[email protected]>

Author: kibanamachine

x-pack/platform/plugins/shared/task_manager/server/lib/get_next_run_at.test.ts

@@ -11,8 +11,7 @@ import { getNextRunAt } from './get_next_run_at';
 import { loggerMock } from '@kbn/logging-mocks';
 const mockLogger = loggerMock.create();
 
-// Failing: See https://github.com/elastic/kibana/issues/220501
-describe.skip('getNextRunAt', () => {
+describe('getNextRunAt', () => {
   afterEach(() => {
     jest.clearAllMocks();
   });
@@ -65,8 +64,8 @@ describe.skip('getNextRunAt', () => {
     expect(nextRunAt.getTime()).toBeGreaterThanOrEqual(testStart.getTime());
   });
 
-  test('should use the rrule with a fixed time when it is given to calculate the next runAt', () => {
-    const now = new Date();
+  test('should use the rrule with a fixed time when it is given to calculate the next runAt (same day)', () => {
+    const now = new Date('2025-06-30T10:00:00.000Z');
     const testStart = new Date(now.getTime() - 500);
     const testRunAt = new Date(now.getTime() - 1000);
     const nextRunAt = getNextRunAt(
@@ -86,23 +85,33 @@ describe.skip('getNextRunAt', () => {
       0,
       mockLogger
     );
+    const expectedNextRunAt = new Date('2025-06-30T12:15:59.500Z');
+    expect(nextRunAt).toEqual(expectedNextRunAt);
+  });
 
-    const currentDay = testStart.getUTCDay();
-    const currentHour = testStart.getUTCHours();
-    const currentSecond = testStart.getUTCSeconds();
-    const currentMilliseconds = testStart.getUTCMilliseconds();
-
-    if (currentHour < 12) {
-      expect(nextRunAt.getUTCDay()).toBe(currentDay);
-    } else {
-      const tomorrow = new Date();
-      tomorrow.setDate(tomorrow.getDate() + 1);
-      expect(nextRunAt.getUTCDay()).toBe(tomorrow.getUTCDay());
-    }
-    expect(nextRunAt.getUTCHours()).toBe(12);
-    expect(nextRunAt.getUTCMinutes()).toBe(15);
-    expect(nextRunAt.getUTCSeconds()).toBe(currentSecond);
-    expect(nextRunAt.getUTCMilliseconds()).toBe(currentMilliseconds);
+  test('should use the rrule with a fixed time when it is given to calculate the next runAt (next day)', () => {
+    const now = new Date('2025-06-30T13:00:00.000Z');
+    const testStart = new Date(now.getTime() - 500);
+    const testRunAt = new Date(now.getTime() - 1000);
+    const nextRunAt = getNextRunAt(
+      taskManagerMock.createTask({
+        schedule: {
+          rrule: {
+            freq: 3, // Daily
+            interval: 1,
+            tzid: 'UTC',
+            byhour: [12],
+            byminute: [15],
+          },
+        },
+        runAt: testRunAt,
+        startedAt: testStart,
+      }),
+      0,
+      mockLogger
+    );
+    const expectedNextRunAt = new Date('2025-07-01T12:15:59.500Z');
+    expect(nextRunAt).toEqual(expectedNextRunAt);
   });
 
   test('should use the rrule with a basic interval time when it is given to calculate the next runAt', () => {