diff --git a/strftime.js b/strftime.js
index 37635ab..c05c3f0 100644
--- a/strftime.js
+++ b/strftime.js
@@ -595,8 +595,10 @@
             else
                 weekday--;
         }
-        var firstDayOfYear = new Date(date.getFullYear(), 0, 1),
-            yday = (date - firstDayOfYear) / 86400000,
+
+        var firstDayOfYearUtc = Date.UTC(date.getFullYear(), 0, 1),
+            dateUtc = Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()),
+            yday = Math.floor((dateUtc - firstDayOfYearUtc) / 86400000),
             weekNum = (yday + 7 - weekday) / 7;
 
         return Math.floor(weekNum);