diff --git a/brmsaptool.scm b/brmsaptool.scm
index 3cdae46..1817811 100644
--- a/brmsaptool.scm
+++ b/brmsaptool.scm
@@ -229,8 +229,27 @@
 	(lambda (a b)
 	  (month<? (cdr a) (cdr b)))))
 
+;; Converts list of start/stop markers to list of pairs of months -
+;; periods.
 (define (period-markers->periods l)
-  l)
+  (let loop ((l l)
+	     (ps '())
+	     (cb #f))
+    (if (null? l)
+	(if cb
+	    (reverse (cons (cons cb #f) ps))
+	    (reverse ps))
+	(let ((m (car l))
+	      (rmt (if cb 'stop 'start)))
+	  (if (eq? (car m) rmt)
+	      (if cb
+		  (loop (cdr l)
+			(cons (cons cb (cdr m)) ps)
+			#f)
+		  (loop (cdr l)
+			ps
+			(cdr m)))
+	      (error 'period-markers->periods "Invalid start/stop sequence marker" m))))))
 
 (define (period-duration p)
   1)
@@ -243,6 +262,15 @@
   (unit-test 'sort-period-markers
 	     (equal? (sort-period-markers '((start 2023 1) (stop 2022 10) (start 2022 3)))
 		     '((start 2022 3) (stop 2022 10) (start 2023 1))))
+  (unit-test 'period-markers->periods
+	     (equal? (period-markers->periods '((start 2022 3) (stop 2022 10) (start 2023 1) (stop 2023 4)))
+		     '(((2022 3) . (2022 10))
+		       ((2023 1) . (2023 4)))))
+  (unit-test 'period-markers->periods-open
+	     (equal? (period-markers->periods '((start 2022 3) (stop 2022 10) (start 2023 1) (stop 2023 4) (start 2023 5)))
+		     '(((2022 3) . (2022 10))
+		       ((2023 1) . (2023 4))
+		       ((2023 5) . #f))))
   (print " ok."))
 
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