--- a/calendar/base/modules/calProviderUtils.jsm
+++ b/calendar/base/modules/calProviderUtils.jsm
@@ -318,18 +318,18 @@ cal.fromRFC3339 = function fromRFC3339(a
 
     if (!dateTime.isDate) {
         dateTime.hour = matches[5];
         dateTime.minute = matches[6];
         dateTime.second = matches[7];
     }
 
     // Timezone handling
-    if (matches[9] == "Z") {
-        // If the dates timezone is "Z", then this is UTC, no matter
+    if (matches[9] == "Z" || matches[9] == "z") {
+        // If the dates timezone is "Z" or "z", then this is UTC, no matter
         // what timezone was passed
         dateTime.timezone = cal.UTC();
 
     } else if (matches[9] == null) {
         // We have no timezone info, only a date. We have no way to
         // know what timezone we are in, so lets assume we are in the
         // timezone of our local calendar, or whatever was passed.