Fix a daylight saving time bug in formatDay.

If the current date-time is 23:MM PST on the day daylight saving time ends, and the passed in date-time is also 23:MM PST on the day daylight saving time ends (possibly for some other MM), then `formatDay` returns 'Tomorrow' rather than 'Today'. See the explanation in the comment added in this CL for why (and for the explanation of the fix).
RELNOTES: Fix a daylight saving time bug in `formatDay`.
PiperOrigin-RevId: 417926514
Change-Id: I2eefc039f16584abcbf4b321349dd18918805995

Author: Closure Team

closure/goog/date/relative.js

@@ -395,13 +395,29 @@ goog.date.relative.formatPast = function(dateMs) {
 goog.date.relative.formatDay = function(dateMs, opt_formatter) {
   'use strict';
   var today = new Date(goog.now());
+  const originalTimezoneOffset = today.getTimezoneOffset();
 
   today.setHours(0);
   today.setMinutes(0);
   today.setSeconds(0);
   today.setMilliseconds(0);
 
-  var dayOffset = (dateMs - today.getTime()) / goog.date.relative.DAY_MS_;
+  // It is possible for the time zone to differ between 00:00 and HH:MM on a
+  // given day if daylight saving time ended on that day some time before HH:MM.
+  // In this case, the number of hours between 00:MM and HH:MM is not HH. It is
+  // HH + 1. In most cases this doesn't matter, but if the current date-time is
+  // 23:MM in PST on the day daylight saving time ended (e.g. Nov 7, 2021) and
+  // `dateMs` is in that same hour, then without correction, the number of hours
+  // between 00:00 and 23:MM would be calculated as 24+ hours, causing
+  // date-times corresponding to 'Today' to be formatted as 'Tomorrow' (e.g.
+  // b/205512072). Here we correct the offset by computing the difference
+  // between today's original time zone and the time zone at 00:00.
+  const timezoneOffsetCorrection =
+      (today.getTimezoneOffset() - originalTimezoneOffset) *
+      goog.date.relative.MINUTE_MS_;
+
+  let dayOffset = (dateMs - today.getTime() + timezoneOffsetCorrection) /
+      goog.date.relative.DAY_MS_;
 
   dayOffset = Math.floor(dayOffset);
 

closure/goog/date/relativecommontests.js

@@ -74,12 +74,10 @@ function timestamp(str) {
 }
 
 testSuite({
-  setUpPage() {
+  setUp() {
     // Ensure goog.now returns a constant timestamp.
     goog.now = () => baseTime;
-  },
 
-  setUp() {
     stubs.replace(goog, 'LOCALE', 'en-US');
     /** @suppress {missingRequire} */
     stubs.set(goog.i18n, 'NumberFormatSymbols', NumberFormatSymbols_en);
@@ -209,6 +207,21 @@ testSuite({
     assertEquals(expected, fn(timestamp('1 January 2010 01:22:11'), format));
   },
 
+  testFormatDay_daylightSavingTime() {
+    // November 7th 2021, 11:57:16 pm PST-08:00 (the day daylight saving time
+    // ended that year, at a time after it ended that day)
+    const daylightSavingEndMs = 1636358236277;
+    goog.now = () => daylightSavingEndMs;
+    assertEquals('today', relative.formatDay(daylightSavingEndMs));
+
+    // March 14, 2021 11:12:34 pm PDT-07:00 (end of the day DST began)
+    const daylightSavingStartMs = 1615788754000;
+    // March 15, 2021 00:12:34 am PDT-07:00 (the day _after_ DST began)
+    const nextDayMs = daylightSavingStartMs + 3600000;
+    goog.now = () => daylightSavingStartMs;
+    assertEquals('tomorrow', relative.formatDay(nextDayMs));
+  },
+
   testGetDateString() {
     const fn = relative.getDateString;