From f9e16964e160f94090d82242a2d02878a9165896 Mon Sep 17 00:00:00 2001
From: Eric Eastwood <contact@ericeastwood.com>
Date: Tue, 18 Oct 2022 18:36:13 -0500
Subject: [PATCH] Fix off-by-one calendar months with time zones that are
 greater than UTC+0 (#86)

Fix off-by-one calendar months with time zones that are greater than UTC+0 (GMT+0).

Fix https://github.com/matrix-org/matrix-public-archive/issues/77

Previously, we would calculate `lastDayOfTheMonthDate` in the local timezone because we were using the vanilla `new Date(year, month, 0)` constructor. For any timezone greater than `UTC+0` (like London UTC+1 or Korea UTC +9), this means that the date is a day-behind when we go back to UTC+0.

**Before:**

```
inputDate Fri, 29 Jul 2022 00:00:00 GMT 1659052800000
lastDayOfTheMonthDate Sat, 30 Jul 2022 23:00:00 GMT 1659222000000
lastDayOfTheMonth 30
```

**After**

```
inputDate Fri, 29 Jul 2022 00:00:00 GMT 1659052800000
lastDayOfTheMonthDate Sun, 31 Jul 2022 00:00:00 GMT 1659225600000
lastDayOfTheMonth 31
```
---
 shared/views/CalendarView.js | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/shared/views/CalendarView.js b/shared/views/CalendarView.js
index 98cc06f..c650317 100644
--- a/shared/views/CalendarView.js
+++ b/shared/views/CalendarView.js
@@ -12,13 +12,18 @@ function sameDay(date1, date2) {
   );
 }
 
-// Month in JavaScript is 0-indexed (January is 0, February is 1, etc),
-// but by using 0 as the day it will give us the last day of the prior
-// month.
+// Get the number of days in the given month where the `inputDate` lies.
 //
 // via https://stackoverflow.com/a/1184359/796832
-function numDaysInMonthForDate(date) {
-  return new Date(date.getUTCFullYear(), date.getUTCMonth() + 1, 0).getUTCDate();
+function numDaysInMonthForDate(inputDate) {
+  // Month in JavaScript is 0-indexed (January is 0, February is 1, etc),
+  // but by using 0 as the day it will give us the last day of the prior
+  const lastDayOfTheMonthDate = new Date(
+    Date.UTC(inputDate.getUTCFullYear(), inputDate.getUTCMonth() + 1, 0)
+  );
+  const lastDayNumberOfTheMonth = lastDayOfTheMonthDate.getUTCDate();
+  // The last day number in the month is a proxy for how many days there are in that month
+  return lastDayNumberOfTheMonth;
 }
 
 // Map from day of week to the localized name of the day
@@ -147,9 +152,10 @@ class CalendarView extends TemplateView {
               }),
               (() => {
                 const todayTs = Date.now();
+                const numberOfDaysInMonth = numDaysInMonthForDate(calendarDate);
 
                 let dayNodes = [];
-                for (let i = 0; i < numDaysInMonthForDate(calendarDate); i++) {
+                for (let i = 0; i < numberOfDaysInMonth; i++) {
                   const dayNumberDate = new Date(calendarDate);
                   // Date is a 1-based number
                   dayNumberDate.setUTCDate(i + 1);