diff --git a/config/version_win.pl b/config/version_win.pl
index 094530e1b20f8..5b84c675be8a3 100755
--- a/config/version_win.pl
+++ b/config/version_win.pl
@@ -54,8 +54,8 @@ sub daysFromBuildID
     $d || die("Unrecognized buildid string.");
 
     my $secondstodays = 60 * 60 * 24;
-    return (POSIX::mktime(00, 00, 00, $d, $m, $y - 1900) -
-            POSIX::mktime(00, 00, 00, 01, 01, 100)) / $secondstodays;
+    return (POSIX::mktime(00, 00, 00, $d, $m - 1, $y - 1900) -
+            POSIX::mktime(00, 00, 00, 01, 00, 100)) / $secondstodays;
 }
 
 #Creates version resource file
diff --git a/js/src/config/version_win.pl b/js/src/config/version_win.pl
index 094530e1b20f8..5b84c675be8a3 100644
--- a/js/src/config/version_win.pl
+++ b/js/src/config/version_win.pl
@@ -54,8 +54,8 @@ sub daysFromBuildID
     $d || die("Unrecognized buildid string.");
 
     my $secondstodays = 60 * 60 * 24;
-    return (POSIX::mktime(00, 00, 00, $d, $m, $y - 1900) -
-            POSIX::mktime(00, 00, 00, 01, 01, 100)) / $secondstodays;
+    return (POSIX::mktime(00, 00, 00, $d, $m - 1, $y - 1900) -
+            POSIX::mktime(00, 00, 00, 01, 00, 100)) / $secondstodays;
 }
 
 #Creates version resource file