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diff --git a/comm/calendar/base/src/CalWeekInfoService.jsm b/comm/calendar/base/src/CalWeekInfoService.jsm
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+++ b/comm/calendar/base/src/CalWeekInfoService.jsm
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+var EXPORTED_SYMBOLS = ["CalWeekInfoService"];
+
+var { XPCOMUtils } = ChromeUtils.importESModule("resource://gre/modules/XPCOMUtils.sys.mjs");
+
+const SUNDAY = 0;
+const THURSDAY = 4;
+
+const lazy = {};
+
+XPCOMUtils.defineLazyPreferenceGetter(lazy, "startWeekday", "calendar.week.start", SUNDAY);
+
+function CalWeekInfoService() {
+  this.wrappedJSObject = this;
+}
+CalWeekInfoService.prototype = {
+  QueryInterface: ChromeUtils.generateQI(["calIWeekInfoService"]),
+  classID: Components.ID("{6877bbdd-f336-46f5-98ce-fe86d0285cc1}"),
+
+  // calIWeekInfoService:
+  getWeekTitle(aDateTime) {
+    /**
+     * This implementation is based on the ISO 8601 standard.
+     * ISO 8601 defines week one as the first week with at least 4
+     * days, and defines Monday as the first day of the week.
+     * Equivalently, the week one is the week with the first Thursday.
+     *
+     * This implementation uses the second definition, because it
+     * enables the user to set a different start-day of the week
+     * (Sunday instead of Monday is a common setting).  If the first
+     * definition was used, all week-numbers could be off by one
+     * depending on the week start day.  (For example, if weeks start
+     * on Sunday, a year that starts on Thursday has only 3 days
+     * [Thu-Sat] in that week, so it would be part of the last week of
+     * the previous year, but if weeks start on Monday, the year would
+     * have four days [Thu-Sun] in that week, so it would be counted
+     * as week 1.)
+     */
+
+    // The week number is the number of days since the start of week 1,
+    // divided by 7 and rounded up. Week 1 is the week containing the first
+    // Thursday of the year.
+    // Thus, the week number of any day is the same as the number of days
+    // between the Thursday of that week and the Thursday of week 1, divided
+    // by 7 and rounded up. (This takes care of days at end/start of a year
+    // which may be part of first/last week in the other year.)
+    // The Thursday of a week is the Thursday that follows the first day
+    // of the week.
+    // The week number of a day is the same as the week number of the first
+    // day of the week. (This takes care of days near the start of the year,
+    // which may be part of the week counted in the previous year.) So we
+    // need the startWeekday.
+
+    // The number of days since the start of the week.
+    // Notice that the result of the subtraction might be negative.
+    // We correct for that by adding 7, and then using the remainder operator.
+    let sinceStartOfWeek = (aDateTime.weekday - lazy.startWeekday + 7) % 7;
+
+    // The number of days to Thursday is the difference between Thursday
+    // and the start-day of the week (again corrected for negative values).
+    let startToThursday = (THURSDAY - lazy.startWeekday + 7) % 7;
+
+    // The yearday number of the Thursday this week.
+    let thisWeeksThursday = aDateTime.yearday - sinceStartOfWeek + startToThursday;
+
+    if (thisWeeksThursday < 1) {
+      // For the first few days of the year, we still are in week 52 or 53.
+      let lastYearDate = aDateTime.clone();
+      lastYearDate.year -= 1;
+      thisWeeksThursday += lastYearDate.endOfYear.yearday;
+    } else if (thisWeeksThursday > aDateTime.endOfYear.yearday) {
+      // For the last few days of the year, we already are in week 1.
+      thisWeeksThursday -= aDateTime.endOfYear.yearday;
+    }
+
+    let weekNumber = Math.ceil(thisWeeksThursday / 7);
+    return weekNumber;
+  },
+
+  /**
+   * gets the first day of a week of a passed day under consideration
+   * of the preference setting "calendar.week.start"
+   *
+   * @param aDate     a date time object
+   * @returns a dateTime-object denoting the first day of the week
+   */
+  getStartOfWeek(aDate) {
+    let date = aDate.clone();
+    date.isDate = true;
+    let offset = lazy.startWeekday - aDate.weekday;
+    date.day += offset;
+    if (offset > 0) {
+      date.day -= 7;
+    }
+    return date;
+  },
+
+  /**
+   * gets the last day of a week of a passed day under consideration
+   * of the preference setting "calendar.week.start"
+   *
+   * @param aDate     a date time object
+   * @returns a dateTime-object denoting the last day of the week
+   */
+  getEndOfWeek(aDate) {
+    let date = this.getStartOfWeek(aDate);
+    date.day += 6;
+    return date;
+  },
+};