diff --git a/calendar/resources/content/calendarUtils.js b/calendar/resources/content/calendarUtils.js
index 48e2ea8f048c..c5f1ff02c243 100644
--- a/calendar/resources/content/calendarUtils.js
+++ b/calendar/resources/content/calendarUtils.js
@@ -303,6 +303,14 @@ function guessSystemTimezone()
     }
     try {
         var stringBundleTZ = gCalendarBundle.getString("likelyTimezone");
+
+        if (stringBundleTZ.indexOf("/mozilla.org/") == -1) {
+            // This happens if the l10n team didn't know how to get a time from
+            // tzdata.c.  To convert an Olson time to a ics-timezone-string we
+            // need to append this prefix.
+            stringBundleTZ = "/mozilla.org/20050126_1/" + stringBundleTZ;
+        }
+
         switch (checkTZ(stringBundleTZ)) {
             case 0: break;
             case 1: 
diff --git a/calendar/resources/locale/en-US/calendar.properties b/calendar/resources/locale/en-US/calendar.properties
index e970c226dccf..57652f805d73 100644
--- a/calendar/resources/locale/en-US/calendar.properties
+++ b/calendar/resources/locale/en-US/calendar.properties
@@ -208,7 +208,9 @@ readOnlyMode=There has been an error reading data for calendar: %1$S. It has bee
 minorError=There has been an error reading data for calendar: %1$S.  However, this error is believed to be minor, so the program will attempt to continue.
 stillReadOnlyError=There has been an error reading data for calendar: %1$S.
 
-# Translators, please put the most likely timezone that the people using your
-# locale will be in.  This needs to be a long name taken from
-# mozilla/calendar/base/src/tzdata.c
-likelyTimezone=/mozilla.org/20050126_1/America/New_York
+# LOCALIZATION NOTE (likelyTimezone):
+#    Translators, please put the most likely timezone that the people using your
+#    locale will be in.  Ideally, this would be a long name taken from
+#    mozilla/calendar/base/src/tzdata.c.  If you don't understand that file or
+#    these instructions, simply put the Olson-timezone name in English, ie "America/New_York"
+likelyTimezone=America/New_York