diff --git a/mozilla/calendar/timezones/Makefile.in b/mozilla/calendar/timezones/Makefile.in
index 33c9ba95ecc..cf3e29da241 100644
--- a/mozilla/calendar/timezones/Makefile.in
+++ b/mozilla/calendar/timezones/Makefile.in
@@ -100,4 +100,20 @@ endif
 libs::
 	$(NSINSTALL) $(TIMEZONES_SQLITE) $(FINAL_TARGET)
 
+# Get all directories in /l10n/*, that contain a calendar subdir.
+calendar_locales := $(subst /calendar,,$(subst $(topsrcdir)/../l10n/,,$(wildcard $(topsrcdir)/../l10n/*/calendar)))
+# Now make sure those dirs are in shipped-locales
+shipped_locales :=$(shell cat $(topsrcdir)/calendar/locales/shipped-locales)
+locales_to_build :=$(foreach locale,$(calendar_locales),$(if $(filter $(locale),$(shipped_locales)),$(locale)))
+
+ifneq ($(strip $(locales_to_build)),)
+# The l10n directory is checked out and contains supported locales, start building our locales
+
+libs:: $(addprefix libs-,$(locales_to_build))
+libs-%:
+	@echo - locales to build -- $(locales_to_build)
+	$(MAKE) -C locales libs AB_CD=$* XPI_NAME=calendar-timezones USE_EXTENSION_MANIFEST=1 NO_JAR_AUTO_REG=1
+
+endif
+
 include $(topsrcdir)/config/rules.mk