docs: note the now/today local-time behavior change in UPDATING.md

Adds an UPDATING.md entry per mistercrunch's review comment, flagging
that charts relying on "Now"/"Today" custom time range anchors will
compute a different (correct) timestamp after this fix, in case any
existing chart configs were tuned around the old UTC-offset bug.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

UPDATING.md

@@ -630,6 +630,12 @@ Added a new combined datasource list endpoint at `GET /api/v1/datasource/` to se
 - Semantic views are included only when the `SEMANTIC_LAYERS` feature flag is enabled.
 - The endpoint enforces strict `order_column` validation and returns `400` for invalid sort columns.
 
+### Custom time range "Now"/"Today" anchors resolve in local time
+
+Custom time ranges that use the "Now" or "Today" anchor (for the Start, End, or the relative anchor itself) previously resolved that anchor in UTC before formatting it into a naive datetime string, which was then re-parsed elsewhere as local time. For users outside UTC, this made the resolved anchor drift by their browser's UTC offset. "Now"/"Today" now resolve directly in local time, matching the later local re-parse.
+
+Charts and dashboards using these anchors will compute a different (correct) timestamp after upgrading; if a chart's filters or drill-downs were tuned to compensate for the old offset, review them after upgrading.
+
 ## 6.1.0
 
 ### ClickHouse minimum driver version bump

superset-frontend/src/explore/components/controls/DateFilterControl/tests/dateParser.test.ts

@@ -0,0 +1,53 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+import { dttmToDayjs } from 'src/explore/components/controls/DateFilterControl/utils/dateParser';
+
+// jest.config.js pins TZ to America/New_York (UTC-4 in June) for all unit
+// tests, giving these assertions a fixed, non-zero UTC offset to catch
+// UTC/local mix-ups against.
+
+afterEach(() => {
+  jest.useRealTimers();
+});
+
+test('dttmToDayjs("now") resolves to the current local time, not UTC', () => {
+  jest.useFakeTimers();
+  // Midnight UTC on June 3rd is 8pm the previous day in America/New_York
+  // (UTC-4 during summer time).
+  jest.setSystemTime(new Date('2024-06-03T00:00:00Z'));
+
+  const result = dttmToDayjs('now');
+
+  // dttmToDayjs's fallback branch (`extendedDayjs(dttm)`) re-parses a naive
+  // "YYYY-MM-DDTHH:mm:ss" string -- the format produced when "now" gets
+  // resolved and formatted elsewhere in this module -- as local wall-clock
+  // time. For that round trip to reproduce the actual current moment,
+  // resolving "now" must also use local wall-clock time rather than UTC.
+  expect(result.format('YYYY-MM-DD HH:mm:ss')).toEqual('2024-06-02 20:00:00');
+});
+
+test('dttmToDayjs("today") resolves to local midnight, not UTC midnight', () => {
+  jest.useFakeTimers();
+  // 2am UTC on June 3rd is still June 2nd, 10pm, in America/New_York.
+  jest.setSystemTime(new Date('2024-06-03T02:00:00Z'));
+
+  const result = dttmToDayjs('today');
+
+  expect(result.format('YYYY-MM-DD HH:mm:ss')).toEqual('2024-06-02 00:00:00');
+});

superset-frontend/src/explore/components/controls/DateFilterControl/utils/dateParser.ts

@@ -40,11 +40,18 @@ export const ISO8601_AND_CONSTANT = RegExp(
 const SPECIFIC_MODE = ['specific', 'today', 'now'];
 
 export const dttmToDayjs = (dttm: string): Dayjs => {
+  // "now"/"today" are later formatted (see dttmToString) into a naive
+  // "YYYY-MM-DDTHH:mm:ss" string with no UTC offset, and that string is
+  // re-parsed elsewhere as local time (the `extendedDayjs(dttm)` fallback
+  // below). Resolving these constants in UTC rather than local time would
+  // make that round trip drift by the browser's UTC offset, which is what
+  // caused the "Now" anchor to display and query the wrong wall-clock time
+  // for users outside UTC.
   if (dttm === 'now') {
-    return extendedDayjs().utc().startOf('second');
+    return extendedDayjs().startOf('second');
   }
   if (dttm === 'today') {
-    return extendedDayjs().utc().startOf('day');
+    return extendedDayjs().startOf('day');
   }
   return extendedDayjs(dttm);
 };