From 8eb4a5e455840bb8c8312e416f41cccbd5aa18e2 Mon Sep 17 00:00:00 2001
From: Marks Polakovs <marks@markspolakovs.me>
Date: Tue, 26 Sep 2023 17:23:10 +0100
Subject: [PATCH] fix(getWeeksBetween): correctly handle differing timezones

---
 src/calendar.ts | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/calendar.ts b/src/calendar.ts
index 287afab..5a1ff44 100644
--- a/src/calendar.ts
+++ b/src/calendar.ts
@@ -369,7 +369,12 @@ export const academicYears: AcademicYear[] = [
     }
 ];
 
-export function getWeeksBetween(startDate: Date, endDate: Date): number {
+export function getWeeksBetween(rawStartDate: Date, endDate: Date): number {
+    // startDate and endDate might not be in the same timezone (in particular around the BST transition)
+    let startDate = rawStartDate;
+    if (startDate.getTimezoneOffset() != endDate.getTimezoneOffset()) {
+        startDate = dayjs(startDate).add(startDate.getTimezoneOffset() - endDate.getTimezoneOffset(), 'minute').toDate();
+    }
     return Math.abs(dayjs(endDate).diff(dayjs(startDate), 'weeks'));
 }