fix(client-core): Isolate internalDayjs instance to prevent global dayjs pollution

- Register custom locale 'cube-internal-en' with weekStart: 1
- Restore original global locale after registration
- Prevents internalDayjs from affecting global dayjs instance
- Add comprehensive test suite for dayjs instance isolation
- Fixes issue where calling internalDayjs changed global week start

Closes issue where customizing internal dayjs instance affected
the default dayjs instance behavior (weekStart changed from 0 to 1)

Author: Debjit Mandal

packages/cubejs-client-core/src/time.ts

@@ -8,6 +8,16 @@ dayjs.extend(quarterOfYear);
 dayjs.extend(duration);
 dayjs.extend(isoWeek);
 
+// A custom locale for internal use that doesn't affect the global dayjs instance
+const cubeInternalLocale = 'cube-internal-en';
+const currentGlobalLocale = dayjs.locale();
+dayjs.locale({
+  ...en,
+  name: cubeInternalLocale,
+  weekStart: 1
+});
+dayjs.locale(currentGlobalLocale);
+
 export type SqlInterval = string;
 
 // TODO: Define a better type as unitOfTime.DurationConstructor in moment.js
@@ -59,7 +69,7 @@ export const DEFAULT_GRANULARITY = 'day';
 
 // When granularity is week, weekStart Value must be 1. However, since the client can change it globally
 // (https://day.js.org/docs/en/i18n/changing-locale) So the function below has been added.
-export const internalDayjs = (...args: any[]): dayjs.Dayjs => dayjs(...args).locale({ ...en, weekStart: 1 });
+export const internalDayjs = (...args: any[]): dayjs.Dayjs => dayjs(...args).locale(cubeInternalLocale);
 
 export const TIME_SERIES: Record<string, (range: DayRange) => string[]> = {
   day: (range) => range.by('d').map(d => d.format('YYYY-MM-DDT00:00:00.000')),

packages/cubejs-client-core/test/dayjs-isolation.test.ts

@@ -0,0 +1,52 @@
+/* globals describe,test,expect */
+
+import 'jest';
+import dayjs from 'dayjs';
+import { internalDayjs } from '../src/time';
+
+describe('Dayjs Instance Isolation', () => {
+  test('internalDayjs should not affect global dayjs instance week start', () => {
+    const initialWeekStart = dayjs().startOf('week').format('dddd');
+    const cubeDayjs = internalDayjs();
+    expect(cubeDayjs.startOf('week').format('dddd')).toBe('Monday');
+    const afterWeekStart = dayjs().startOf('week').format('dddd');
+    expect(afterWeekStart).toBe(initialWeekStart);
+    expect(afterWeekStart).toBe('Sunday');
+  });
+
+  test('internalDayjs week calculation should use Monday as week start', () => {
+    const testDate = '2024-01-10';
+    
+    const internalWeekStart = internalDayjs(testDate).startOf('week');
+    expect(internalWeekStart.format('YYYY-MM-DD')).toBe('2024-01-08');
+    expect(internalWeekStart.format('dddd')).toBe('Monday');
+    
+    const globalWeekStart = dayjs(testDate).startOf('week');
+    expect(globalWeekStart.format('YYYY-MM-DD')).toBe('2024-01-07');
+    expect(globalWeekStart.format('dddd')).toBe('Sunday');
+  });
+
+  test('multiple calls to internalDayjs should not affect global instance', () => {
+    const initialWeekStart = dayjs().startOf('week').format('dddd');
+    
+    internalDayjs('2024-01-01');
+    internalDayjs('2024-02-01');
+    internalDayjs('2024-03-01');
+    
+    expect(dayjs().startOf('week').format('dddd')).toBe(initialWeekStart);
+  });
+
+  test('internalDayjs should consistently use weekStart: 1', () => {
+    const dates = [
+      '2024-01-10', // Wednesday
+      '2024-02-15', // Thursday
+      '2024-03-20', // Wednesday
+      '2024-04-25', // Thursday
+    ];
+    
+    dates.forEach((date) => {
+      const weekStart = internalDayjs(date).startOf('week');
+      expect(weekStart.format('dddd')).toBe('Monday');
+    });
+  });
+});