unit test dayJSAddWithDSTFixed (#4192)

# What this PR does
unit test dayJSAddWithDSTFixed utility function


## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

grafana-plugin/jest.config.js

@@ -1,7 +1,3 @@
-// force timezone to UTC to allow tests to work regardless of local timezone
-// generally used by snapshots, but can affect specific tests
-process.env.TZ = 'UTC';
-
 const esModules = ['@grafana', 'uplot', 'ol', 'd3', 'react-colorful', 'uuid', 'openapi-fetch'].join('|');
 
 module.exports = {

grafana-plugin/package.json

@@ -11,9 +11,11 @@
     "build:dev": "webpack -c ./webpack.config.ts --env development",
     "labels:link": "yarn --cwd ../../gops-labels/frontend link && yarn link \"@grafana/labels\" && yarn --cwd ../../gops-labels/frontend watch",
     "labels:unlink": "yarn --cwd ../../gops-labels/frontend unlink",
-    "test": "jest --verbose",
-    "test:report": "HTML_REPORT_ENABLED=true jest",
-    "test:silent": "jest --silent",
+    "test-utc": "TZ=UTC jest --verbose --testNamePattern '^((?!@london-tz).)*$'",
+    "test-london-tz": "TZ=Europe/London jest --verbose --testNamePattern '@london-tz'",
+    "test": "yarn test-utc && yarn test-london-tz",
+    "test:report": "HTML_REPORT_ENABLED=true yarn test",
+    "test:silent": "yarn test --silent",
     "test:e2e": "yarn playwright test --grep-invert @expensive",
     "test:e2e-expensive": "yarn playwright test",
     "test:e2e:watch": "yarn test:e2e --ui",

grafana-plugin/src/containers/RotationForm/RotationForm.helpers.test.ts

@@ -1,6 +1,11 @@
-import dayjs from 'dayjs';
+import dayjs, { Dayjs } from 'dayjs';
+import timezone from 'dayjs/plugin/timezone';
+import utc from 'dayjs/plugin/utc';
 
-import { getDateForDatePicker } from './RotationForm.helpers';
+import { dayJSAddWithDSTFixed, getDateForDatePicker } from './RotationForm.helpers';
+
+dayjs.extend(timezone);
+dayjs.extend(utc);
 
 describe('RotationForm helpers', () => {
   describe('getDateForDatePicker()', () => {
@@ -21,4 +26,92 @@ describe('RotationForm helpers', () => {
       expect(result.toString()).toContain('Tue Apr 30 2024');
     });
   });
+
+  describe('dayJSAddWithDSTFixed() @london-tz', () => {
+    it(`corrects resulting hour to be the same as in input if start date is before London DST (GMT + 0) 
+      and resulting date is within London DST (GMT + 1)`, () => {
+      // Base date is out of DST: 20th Mar 3:00 (GMT + 0)
+      const baseDate = dayjs('2018-03-20 3:00');
+
+      // Result is within DST (GMT + 1)
+      const result = dayJSAddWithDSTFixed({
+        baseDate,
+        addParams: [2, 'weeks'],
+      });
+
+      // Check that although DST change happened, hours are the same in UTC
+      expect(baseDate.utc().hour()).toBe(result.utc().hour());
+      expect(result.utc().format()).toBe('2018-04-03T03:00:00Z');
+    });
+
+    it(`corrects resulting hour to be the same as in input if start date is within London DST (GMT + 1) 
+      and resulting date is after London DST (GMT + 0)`, () => {
+      // Base date is within DST: 20th Oct 3:00 (GMT + 1)
+      const baseDate = dayjs('2018-10-20 3:00');
+
+      // Result is out of DST change (GMT + 0)
+      const result = dayJSAddWithDSTFixed({
+        baseDate,
+        addParams: [2, 'weeks'],
+      });
+
+      // Check that although DST change happened, hours are the same in UTC
+      expect(baseDate.utc().hour()).toBe(result.utc().hour());
+      expect(result.utc().format()).toBe('2018-11-03T02:00:00Z');
+    });
+
+    it('does nothing with hours if both start date and resulting date are within London DST', () => {
+      // Base date is within DST: 20th May 3:00 (GMT + 1)
+      const baseDate = dayjs('2018-5-20 3:00');
+
+      [
+        [24, 'hours'],
+        [2, 'weeks'],
+        [1, 'months'],
+      ].forEach((addParams: Parameters<Dayjs['add']>) => {
+        expect(
+          dayJSAddWithDSTFixed({
+            baseDate,
+            addParams,
+          })
+            .utc()
+            .hour()
+        ).toBe(baseDate.utc().hour());
+      });
+    });
+
+    it('does nothing with hours if both start date and resulting date are out of London DST', () => {
+      // Base date is out of DST: 20th Jan 3:00 (GMT + 0)
+      const baseDate = dayjs('2018-1-20 3:00');
+
+      [
+        [24, 'hours'],
+        [2, 'weeks'],
+        [1, 'months'],
+      ].forEach((addParams: Parameters<Dayjs['add']>) => {
+        expect(
+          dayJSAddWithDSTFixed({
+            baseDate,
+            addParams,
+          })
+            .utc()
+            .hour()
+        ).toBe(baseDate.utc().hour());
+      });
+    });
+
+    it('adds hours correctly within the same day', () => {
+      // Base date is out of DST: 20th Jan 3:00 (GMT + 0)
+      const baseDate = dayjs('2018-1-20 3:00');
+
+      expect(
+        dayJSAddWithDSTFixed({
+          baseDate,
+          addParams: [8, 'hours'],
+        })
+          .utc()
+          .hour()
+      ).toBe(baseDate.utc().hour() + 8);
+    });
+  });
 });