diff --git a/src/cal-period.scm b/src/cal-period.scm
index ea1cf3d..e88954c 100644
--- a/src/cal-period.scm
+++ b/src/cal-period.scm
@@ -138,7 +138,10 @@
 			    (not (null? (cdr args))))
 		       (cadr args)
 		       #f)))
-     (list TAG-CAL-PERIOD since before scomment bcomment)))
+     (list TAG-CAL-PERIOD
+	   (if since (cal-ensure-month since) since)
+	   (if before (cal-ensure-month before) before)
+	   scomment bcomment)))
 
  ;; Simple accessors
  (define cal-period-since cadr)