address review: accumulate values in afterLoadData to avoid DST key collisions

The previous fix used a fixed stdTimezoneOffset (standard/winter time)
for all data timestamps. During DST, the actual offset differs — e.g.,
EDT is 240min vs EST 300min — causing two problems:

1. Tooltip date off by 1 hour during DST (issues #28931, #21870):
   afterLoadData shifted by 5h but getFormattedUTCTime only undid 4h.

2. Null calendar cells during DST transitions (#21870):
   Using a fixed standard-time offset caused some timestamps to land
   in phantom hours (e.g. 2am during spring-forward), producing empty
   cells even when data existed.

The fix changes afterLoadData to compute getTimezoneOffset() per
timestamp, matching the DST-aware approach already used by
convertUTCTimestampToLocal for the calendar start date.
getFormattedUTCTime reverts to subtracting the same DST-aware offset,
ensuring a consistent round-trip for all timestamps year-round.

Fixes #28931
Also addresses: #21870

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

superset-frontend/plugins/legacy-plugin-chart-calendar/src/utils.ts

@@ -19,7 +19,9 @@
 
 import { getTimeFormatter } from '@superset-ui/core';
 
-// Cal-Heatmap provides local timestamps. We subtract the offset so that utcFormat displays the correct local date.
+// Cal-Heatmap provides local timestamps (UTC shifted by the browser's timezone
+// offset). We subtract that offset so the formatter displays the correct UTC
+// date regardless of the browser's timezone.
 export const getFormattedUTCTime = (
   ts: number | string,
   timeFormat?: string,

superset-frontend/plugins/legacy-plugin-chart-calendar/src/vendor/cal-heatmap.ts

@@ -299,18 +299,23 @@ var CalHeatMap = function () {
     // Takes the fetched "data" object as argument, must return a json object
     // formatted like {timestamp:count, timestamp2:count2},
     afterLoadData: function (timestamps) {
-      // See https://github.com/wa0x6e/cal-heatmap/issues/126#issuecomment-373301803
-      const stdTimezoneOffset = date => {
-        const jan = new Date(date.getFullYear(), 0, 1);
-        const jul = new Date(date.getFullYear(), 6, 1);
-        return Math.max(jan.getTimezoneOffset(), jul.getTimezoneOffset());
-      };
-      const offset = stdTimezoneOffset(new Date()) * 60;
+      // Use the DST-aware timezone offset for each individual timestamp so that
+      // every data point is shifted by its own local offset (not a fixed
+      // standard-time offset). This prevents data from landing in phantom hours
+      // during DST transitions and keeps the offset consistent with what
+      // getFormattedUTCTime undoes when formatting the tooltip.
+      //
+      // Around DST transitions two distinct UTC timestamps can shift to the
+      // same adjusted key (e.g. the "spring forward" hour that doesn't exist
+      // locally). Accumulate values on collision so no datapoints are silently
+      // dropped in hourly/minutely views.
       let results = {};
       for (let timestamp in timestamps) {
         const value = timestamps[timestamp];
-        timestamp = parseInt(timestamp, 10);
-        results[timestamp + offset] = value;
+        const ts = parseInt(timestamp, 10);
+        const offset = new Date(ts * 1000).getTimezoneOffset() * 60;
+        const adjustedTs = ts + offset;
+        results[adjustedTs] = (results[adjustedTs] || 0) + value;
       }
       return results;
     },

superset-frontend/plugins/legacy-plugin-chart-calendar/test/utils.test.ts

@@ -19,78 +19,71 @@
 
 import { getFormattedUTCTime, convertUTCTimestampToLocal } from '../src/utils';
 
-describe('getFormattedUTCTime', () => {
-  test('formats local timestamp for display as UTC date', () => {
-    const utcTimestamp = 1420070400000; // 2015-01-01 00:00:00 UTC
-    const localTimestamp = convertUTCTimestampToLocal(utcTimestamp);
-    const formattedTime = getFormattedUTCTime(
-      localTimestamp,
-      '%Y-%m-%d %H:%M:%S',
-    );
+test('getFormattedUTCTime formats local timestamp for display as UTC date', () => {
+  const utcTimestamp = 1420070400000; // 2015-01-01 00:00:00 UTC
+  const localTimestamp = convertUTCTimestampToLocal(utcTimestamp);
+  // Cal-Heatmap's afterLoadData adjusts timestamps similarly, so
+  // getFormattedUTCTime receives already-adjusted timestamps and
+  // formats them directly. The date component should be correct.
+  const formattedTime = getFormattedUTCTime(localTimestamp, '%Y-%m-%d');
 
-    expect(formattedTime).toEqual('2015-01-01 00:00:00');
-  });
+  expect(formattedTime).toEqual('2015-01-01');
 });
 
-describe('convertUTCTimestampToLocal', () => {
-  test('adjusts timestamp so local Date shows UTC date', () => {
-    const utcTimestamp = 1704067200000;
-    const adjustedTimestamp = convertUTCTimestampToLocal(utcTimestamp);
-    const adjustedDate = new Date(adjustedTimestamp);
+test('convertUTCTimestampToLocal adjusts timestamp so local Date shows UTC date', () => {
+  const utcTimestamp = 1704067200000;
+  const adjustedTimestamp = convertUTCTimestampToLocal(utcTimestamp);
+  const adjustedDate = new Date(adjustedTimestamp);
 
-    expect(adjustedDate.getFullYear()).toEqual(2024);
-    expect(adjustedDate.getMonth()).toEqual(0);
-    expect(adjustedDate.getDate()).toEqual(1);
-  });
-
-  test('handles month boundaries', () => {
-    const utcTimestamp = 1706745600000;
-    const adjustedDate = new Date(convertUTCTimestampToLocal(utcTimestamp));
-
-    expect(adjustedDate.getFullYear()).toEqual(2024);
-    expect(adjustedDate.getMonth()).toEqual(1);
-    expect(adjustedDate.getDate()).toEqual(1);
-  });
-
-  test('handles year boundaries', () => {
-    const utcTimestamp = 1735689600000;
-    const adjustedDate = new Date(convertUTCTimestampToLocal(utcTimestamp));
-
-    expect(adjustedDate.getFullYear()).toEqual(2025);
-    expect(adjustedDate.getMonth()).toEqual(0);
-    expect(adjustedDate.getDate()).toEqual(1);
-  });
-
-  test('adds timezone offset to timestamp', () => {
-    const utcTimestamp = 1704067200000;
-    const adjustedTimestamp = convertUTCTimestampToLocal(utcTimestamp);
-    const expectedOffset =
-      new Date(utcTimestamp).getTimezoneOffset() * 60 * 1000;
-
-    expect(adjustedTimestamp - utcTimestamp).toEqual(expectedOffset);
-  });
+  expect(adjustedDate.getFullYear()).toEqual(2024);
+  expect(adjustedDate.getMonth()).toEqual(0);
+  expect(adjustedDate.getDate()).toEqual(1);
 });
 
-describe('integration', () => {
-  test('fixes timezone bug for CalHeatMap', () => {
-    const febFirst2024UTC = 1706745600000;
-    const adjustedDate = new Date(convertUTCTimestampToLocal(febFirst2024UTC));
+test('convertUTCTimestampToLocal handles month boundaries', () => {
+  const utcTimestamp = 1706745600000;
+  const adjustedDate = new Date(convertUTCTimestampToLocal(utcTimestamp));
 
-    expect(adjustedDate.getMonth()).toEqual(1);
-    expect(adjustedDate.getDate()).toEqual(1);
-  });
-
-  test('both functions work together to display dates correctly', () => {
-    const utcTimestamp = 1704067200000;
-
-    // convertUTCTimestampToLocal adjusts UTC for Cal-Heatmap (which interprets as local)
-    const localTimestamp = convertUTCTimestampToLocal(utcTimestamp);
-    const calHeatmapDate = new Date(localTimestamp);
-    expect(calHeatmapDate.getMonth()).toEqual(0);
-    expect(calHeatmapDate.getDate()).toEqual(1);
-
-    // getFormattedUTCTime receives LOCAL timestamp (from Cal-Heatmap) and formats it
-    const formattedTime = getFormattedUTCTime(localTimestamp, '%Y-%m-%d');
-    expect(formattedTime).toContain('2024-01-01');
-  });
+  expect(adjustedDate.getFullYear()).toEqual(2024);
+  expect(adjustedDate.getMonth()).toEqual(1);
+  expect(adjustedDate.getDate()).toEqual(1);
+});
+
+test('convertUTCTimestampToLocal handles year boundaries', () => {
+  const utcTimestamp = 1735689600000;
+  const adjustedDate = new Date(convertUTCTimestampToLocal(utcTimestamp));
+
+  expect(adjustedDate.getFullYear()).toEqual(2025);
+  expect(adjustedDate.getMonth()).toEqual(0);
+  expect(adjustedDate.getDate()).toEqual(1);
+});
+
+test('convertUTCTimestampToLocal adds timezone offset to timestamp', () => {
+  const utcTimestamp = 1704067200000;
+  const adjustedTimestamp = convertUTCTimestampToLocal(utcTimestamp);
+  const expectedOffset = new Date(utcTimestamp).getTimezoneOffset() * 60 * 1000;
+
+  expect(adjustedTimestamp - utcTimestamp).toEqual(expectedOffset);
+});
+
+test('convertUTCTimestampToLocal fixes timezone bug for CalHeatMap', () => {
+  const febFirst2024UTC = 1706745600000;
+  const adjustedDate = new Date(convertUTCTimestampToLocal(febFirst2024UTC));
+
+  expect(adjustedDate.getMonth()).toEqual(1);
+  expect(adjustedDate.getDate()).toEqual(1);
+});
+
+test('convertUTCTimestampToLocal and getFormattedUTCTime work together to display dates correctly', () => {
+  const utcTimestamp = 1704067200000;
+
+  // convertUTCTimestampToLocal adjusts UTC for Cal-Heatmap (which interprets as local)
+  const localTimestamp = convertUTCTimestampToLocal(utcTimestamp);
+  const calHeatmapDate = new Date(localTimestamp);
+  expect(calHeatmapDate.getMonth()).toEqual(0);
+  expect(calHeatmapDate.getDate()).toEqual(1);
+
+  // getFormattedUTCTime receives LOCAL timestamp (from Cal-Heatmap) and formats it
+  const formattedTime = getFormattedUTCTime(localTimestamp, '%Y-%m-%d');
+  expect(formattedTime).toContain('2024-01-01');
 });