Datepicker: fixed #4285 - Week of the Year off by one during daylight…

… saving time

And simplified calculation

ui/ui.datepicker.js

@@ -821,20 +821,13 @@ $.extend(Datepicker.prototype, {
 	   @param  date  Date - the date to get the week for
 	   @return  number - the number of the week within the year that contains this date */
 	iso8601Week: function(date) {
-		var checkDate = new Date(date.getFullYear(), date.getMonth(), date.getDate());
-		var firstMon = new Date(checkDate.getFullYear(), 1 - 1, 4); // First week always contains 4 Jan
-		var firstDay = firstMon.getDay() || 7; // Day of week: Mon = 1, ..., Sun = 7
-		firstMon.setDate(firstMon.getDate() + 1 - firstDay); // Preceding Monday
-		if (firstDay < 4 && checkDate < firstMon) { // Adjust first three days in year if necessary
-			checkDate.setDate(checkDate.getDate() - 3); // Generate for previous year
-			return $.datepicker.iso8601Week(checkDate);
-		} else if (checkDate > new Date(checkDate.getFullYear(), 12 - 1, 28)) { // Check last three days in year
-			firstDay = new Date(checkDate.getFullYear() + 1, 1 - 1, 4).getDay() || 7;
-			if (firstDay > 4 && (checkDate.getDay() || 7) < firstDay - 3) { // Adjust if necessary
-				return 1;
-			}
-		}
-		return Math.floor(((checkDate - firstMon) / 86400000) / 7) + 1; // Weeks to given date
+		var checkDate = new Date(date.getTime());
+		// Find Thursday of this week starting on Monday
+		checkDate.setDate(checkDate.getDate() + 4 - (checkDate.getDay() || 7));
+		var time = checkDate.getTime();
+		checkDate.setMonth(0); // Compare with Jan 1
+		checkDate.setDate(1);
+		return Math.floor(Math.round((time - checkDate) / 86400000) / 7) + 1;
 	},
 
 	/* Parse a string value into a date object.