Revert "When creating cal-period, ensure months for periods."

This reverts commit 488499cf23.

src/cal-period.scm

@@ -138,10 +138,7 @@
 			    (not (null? (cdr args))))
 		       (cadr args)
 		       #f)))
-     (list TAG-CAL-PERIOD
-	   (if since (cal-ensure-month since) since)
-	   (if before (cal-ensure-month before) before)
-	   scomment bcomment)))
+     (list TAG-CAL-PERIOD since before scomment bcomment)))
 
  ;; Simple accessors
  (define cal-period-since cadr)