diff --git a/lib/index.js b/lib/index.js
index 5bbb220..51d83e4 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -108,10 +108,12 @@
         case 's': return Math.floor((d.getTime() - msDelta) / 1000);
         case 'T': return _strftime(locale.formats.T || '%H:%M:%S', d, locale);
         case 't': return '\t';
+        case 'U': return pad(weekNumber(d, 'sunday'));
         case 'u':
           var day = d.getDay();
           return day == 0 ? 7 : day; // 1 - 7, Monday is first day of the week
         case 'v': return _strftime(locale.formats.v || '%e-%b-%Y', d, locale);
+        case 'W': return pad(weekNumber(d, 'monday'));
         case 'w': return d.getDay(); // 0 - 6, Sunday is first day of the week
         case 'Y': return d.getFullYear();
         case 'y':
@@ -163,4 +165,26 @@
     return hour;
   }
 
+  // firstWeekday: 'sunday' or 'monday', default is 'sunday'
+  //
+  // Pilfered & ported from Ruby's strftime implementation.
+  function weekNumber(d, firstWeekday) {
+    firstWeekday = firstWeekday || 'sunday';
+
+    // This works by shifting the weekday back by one day if we
+    // are treating Monday as the first day of the week.
+    var wday = d.getDay();
+    if (firstWeekday == 'monday') {
+      if (wday == 0) // Sunday
+        wday = 6;
+      else
+        wday--;
+    }
+    var firstDayOfYear = new Date(d.getFullYear(), 0, 1)
+      , yday = (d - firstDayOfYear) / 86400000
+      , weekNum = (yday + 7 - wday) / 7
+      ;
+    return Math.floor(weekNum);
+  }
+
 }());
diff --git a/test/test.js b/test/test.js
index fe0b84a..cb44643 100644
--- a/test/test.js
+++ b/test/test.js
@@ -15,12 +15,13 @@ assert.fn = function(value, msg) {
   assert.equal('function', typeof value, msg)
 }
 
-assert.format = function(format, expected, expectedUTC) {
+assert.format = function(format, expected, expectedUTC, time) {
+  time = time || Time
   function _assertFmt(expected, name) {
     name = name || 'strftime'
-    var actual = lib[name](format, Time)
+    var actual = lib[name](format, time)
     assert.equal(expected, actual,
-                 name + '("' + format + '", Time) is ' + JSON.stringify(actual)
+                 name + '("' + format + '", ' + time + ') is ' + JSON.stringify(actual)
                  + ', expected ' + JSON.stringify(expected))
   }
 
@@ -64,8 +65,12 @@ assert.format('%s', '1307472705')
 assert.format('%T', null, '18:51:45')
 assert.format('%t', '\t')
 assert.format('%u', '2')
+assert.format('%U', '23')
+assert.format('%U', '24', null, new Date(+Time + 5 * 86400000))
 assert.format('%v', '7-Jun-2011')
 assert.format('%w', '2')
+assert.format('%W', '23')
+assert.format('%W', '23', null, new Date(+Time + 5 * 86400000))
 assert.format('%Y', '2011')
 assert.format('%y', '11')
 assert.format('%Z', null, 'GMT')