fix: Clicking DST date from outside DST and active timezone increments by an hour (fixes #1059)

Author: Jasenkoo

src/VueDatePicker/utils/timezone.ts

@@ -28,11 +28,21 @@ export const sanitizeDateToLocal = (date: MaybeDate, tz?: TimeZoneConfig, reset?
     return tz.exactMatch ? getDateInTz(date, tz, reset) : localToTz(newDate, tz.timezone);
 };
 
+const isDST = (date: Date) => {
+    const january = new Date(date.getFullYear(), 0, 1);
+
+    const januaryOffset = january.getTimezoneOffset();
+    const dateOffset = date.getTimezoneOffset();
+
+    return dateOffset < januaryOffset;
+};
+
 export const getTimezoneOffset = (timezone?: string, localDate?: Date) => {
     if (!timezone) return 0;
     const date = new Date();
     const utcDate = new Date(date.toLocaleString('en-US', { timeZone: 'UTC' }));
     const specificDate = new Date(date.toLocaleString('en-US', { timeZone: timezone }));
-    const dstOffset = (localDate ?? specificDate).getTimezoneOffset() / 60;
+    const dateToOffset = isDST(localDate, timezone) ? specificDate : localDate;
+    const dstOffset = dateToOffset.getTimezoneOffset() / 60;
     return (+utcDate - +specificDate) / (1000 * 60 * 60) - dstOffset;
 };