(js) Fix computation of week number

Fixes #3973, 3976

NEWS

@@ -34,6 +34,7 @@ Bug fixes
  - [web] append ics file extension when importing events (#2308)
  - [web] handle URI in vCard photos (#2683)
  - [web] handle semicolon in values during LDIF import (#1760)
+ - [web] fixed computation of week number (#3973, #3976)
  - [eas] properly skip folders we don't want to synchronize (#3943)
  - [eas] fixed 30 mins freebusy offset with S Planner
  - [eas] now correctly handles reminders on tasks (#3964)

UI/WebServerResources/js/Common/utils.js

@@ -325,6 +325,22 @@ Date.prototype.beginOfDay = function() {
     return beginOfDay;
 };
 
+/**
+ * See [SOGoUser dayOfWeekForDate:]
+ */
+Date.prototype.dayOfWeek = function(localeProvider) {
+  var offset, baseDayOfWeek, dayOfWeek;
+
+  offset = localeProvider.firstDayOfWeek;
+  baseDayOfWeek = this.getDay();
+  if (offset > baseDayOfWeek)
+    baseDayOfWeek += 7;
+
+  dayOfWeek = baseDayOfWeek - offset;
+
+  return dayOfWeek;
+};
+
 /**
  * See [SOGoUser firstWeekOfYearForDate:]
  */
@@ -336,10 +352,10 @@ Date.prototype.firstWeekOfYearForDate = function(localeProvider) {
   januaryFirst = new Date(this.getTime());
   januaryFirst.setMonth(0);
   januaryFirst.setDate(1);
-  dayOfWeek = januaryFirst.getDay();
+  dayOfWeek = januaryFirst.dayOfWeek(localeProvider);
 
   if (firstWeekRule == 'First4DayWeek') {
-    if ((dayOfWeek + localeProvider.firstDayOfWeek) % 7 < 4)
+    if (dayOfWeek < 4)
       firstWeek = januaryFirst.beginOfWeek(localeProvider.firstDayOfWeek);
     else
       firstWeek = januaryFirst.addDays(7).beginOfWeek(localeProvider.firstDayOfWeek);