implement calendarWeeks option

it prepends a .cw cell to each row and displays the current calendar week before the first day of the week
defaults weekStart to monday
specs and basic styling included

js/bootstrap-datepicker.js

@@ -108,7 +108,8 @@
 		this.todayBtn = (options.todayBtn||this.element.data('date-today-btn')||false);
 		this.todayHighlight = (options.todayHighlight||this.element.data('date-today-highlight')||false);
 
-		this.weekStart = ((options.weekStart||this.element.data('date-weekstart')||dates[this.language].weekStart||0) % 7);
+		this.calendarWeeks = !!options.calendarWeeks;
+		this.weekStart = ((options.weekStart||this.element.data('date-weekstart')||this.calendarWeeks||dates[this.language].weekStart||0) % 7);
 		this.weekEnd = ((this.weekStart + 6) % 7);
 		this.startDate = -Infinity;
 		this.endDate = Infinity;
@@ -339,6 +340,11 @@
 		fillDow: function(){
 			var dowCnt = this.weekStart,
 			html = '<tr>';
+			if(this.calendarWeeks){
+				var cell = '<th class="cw">&nbsp;</th>';
+				html += cell;
+				this.picker.find('.datepicker-days thead tr:first-child').prepend(cell);
+			}
 			while (dowCnt < this.weekStart + 7) {
 				html += '<th class="dow">'+dates[this.language].daysMin[(dowCnt++)%7]+'</th>';
 			}
@@ -365,7 +371,7 @@
 				endMonth = this.endDate !== Infinity ? this.endDate.getUTCMonth() : Infinity,
 				currentDate = this.date && this.date.valueOf(),
 				today = new Date();
-			this.picker.find('.datepicker-days thead th:eq(1)')
+			this.picker.find('.datepicker-days thead th.switch')
 						.text(dates[this.language].months[month]+' '+year);
 			this.picker.find('tfoot th.today')
 						.text(dates[this.language].today)
@@ -384,6 +390,13 @@
 			while(prevMonth.valueOf() < nextMonth) {
 				if (prevMonth.getUTCDay() == this.weekStart) {
 					html.push('<tr>');
+					if(this.calendarWeeks){
+                        // from https://github.com/timrwood/moment/blob/master/moment.js#L128
+						var a = new Date(prevMonth.getYear(), prevMonth.getMonth(), prevMonth.getDate() - prevMonth.getDay() + 5),
+							b = new Date(a.getFullYear(), 0, 4),
+							calWeek =  ~~((a - b) / 864e5 / 7 + 1.5);
+						html.push('<td class="cw">'+ calWeek +'</td>')
+					}
 				}
 				clsName = '';
 				if (prevMonth.getUTCFullYear() < year || (prevMonth.getUTCFullYear() == year && prevMonth.getUTCMonth() < month)) {

less/datepicker.less

@@ -154,6 +154,19 @@
 	/*.dow {
 		border-top: 1px solid #ddd !important;
 	}*/
+
+	// Basic styling for calendar-week cells
+	.cw {
+		font-size: 10px;
+		width: 12px;
+		padding: 0 2px 0 5px;
+	}
+	thead tr:first-child th.cw {
+		cursor: default;
+	}
+	thead tr:first-child th.cw {
+		background-color: transparent;
+	}
 }
 .input-append,
 .input-prepend {

tests/suites/calendar-weeks.js

@@ -0,0 +1,52 @@
+module('Calendar Weeks', {
+    setup: function(){
+        this.input = $('<input type="text">')
+            .appendTo('#qunit-fixture')
+            .val('2013-01-14')
+            .datepicker({
+                format: 'yyyy-mm-dd',
+                calendarWeeks: true
+            })
+            .focus(); // Activate for visibility checks
+        this.dp = this.input.data('datepicker')
+        this.picker = this.dp.picker;
+    },
+    teardown: function(){
+        this.picker.remove();
+    }
+});
+
+test('weekStart defaults to monday', function(){
+    equal(this.dp.weekStart, 1, 'Week start defaults to monday');
+});
+
+test('adds cw header column', function(){
+    var target = this.picker.find('.datepicker-days thead th:first-child');
+    ok(target.hasClass('cw'), 'First column heading is from cw column');
+});
+
+test('adds calendar week cells to each day row', function(){
+    var target = this.picker.find('.datepicker-days tbody tr');
+
+    expect(target.length);
+    target.each(function(i){
+        var t = $(this).children().first();
+        ok(t.hasClass('cw'), "First column is cw column");
+    });
+});
+
+test('displays correct calendar week', function(){
+    var target = this.picker.find('.datepicker-days tbody tr');
+
+    expect(target.length);
+    target.each(function(i){
+        var t = $(this).children().first();
+        equal(t.text(), i+1, "Displays correct calendar weeks");
+    });
+});
+
+test('it prepends column to switcher thead row', function(){
+    var target = this.picker.find('.datepicker-days thead tr:first-child');
+    equal(target.children().length, 4, 'first row has 4 columns');
+    ok(target.children().first().hasClass('cw'), 'cw column is prepended');
+});

tests/tests.html

@@ -34,6 +34,7 @@
         <script src="suites/events.js"></script>
         <script src="suites/options.js"></script>
         <script src="suites/inline.js"></script>
+        <script src="suites/calendar-weeks.js"></script>
     </head>
     <body>
         <h1 id="qunit-header">bootstrap-datepicker</h1>