diff --git a/brmsaptool.scm b/brmsaptool.scm
index 90a7d90..dc0e5b7 100644
--- a/brmsaptool.scm
+++ b/brmsaptool.scm
@@ -26,6 +26,7 @@
 	(chicken file posix)
 	(chicken io)
 	(chicken string)
+	(chicken format)
 
 	(chicken process-context))
 
@@ -50,7 +51,7 @@
   (syntax-rules ()
     ((_ name condition)
      (if (with-handler (lambda (x) #f)
-		       (lambda () condition))
+		       condition)
 	 (display ".")
 	 (error 'unit-test name)))))
 
@@ -131,28 +132,28 @@
        (car m)
        (cdr m)
        (cadr m)
-       (nil? (cddr m))
+       (null? (cddr m))
        (integer? (car m))
        (integer? (cadr m))
-       (>= (car m 1000))
-       (<= (car m 9999))
-       (>= (cadr m 1))
-       (<= (cadr m 12))))
+       (>= (car m) 1000)
+       (<= (car m) 9999)
+       (>= (cadr m) 1)
+       (<= (cadr m) 12)))
 
 ;; Converts string in a format YYYY-MM to valid month. Returns #f if
 ;; the conversion fails.
 (define (string->month s)
   (let ((l (string-split s "-")))
     (if (or (not l)
-	    (nil? l)
-	    (nil? (cdr l))
-	    (not (nil? (cddr l))))
+	    (null? l)
+	    (null? (cdr l))
+	    (not (null? (cddr l))))
 	#f
 	(let ((y (string->number (car l)))
 	      (m (string->number (cadr l))))
 	  (if (and y m)
 	      (let ((M (list y m)))
-		(if (valid-month? M)
+		(if (month-valid? M)
 		    M
 		    #f))
 	      #f)))))
@@ -160,18 +161,18 @@
 ;; Formats (valid) month as YYYY-MM string
 (define (month->string M)
   (if (month-valid? M)
-      (let ((y (car s))
-	    (m (cadr s)))
+      (let ((y (car M))
+	    (m (cadr M)))
 	(sprintf "~A-~A~A"
 		 y
-		 (if (< m 10) " " "")
+		 (if (< m 10) "0" "")
 		 m))
       (error 'string->month "Invalid month" M)))
 
 ;; Returns true if both arguments are a valid month and are equal
 (define (month=? m n)
-  (and (valid-month? m)
-       (valid-month? n)
+  (and (month-valid? m)
+       (month-valid? n)
        (equal? m n)))
 
 ;; Returns true if the first argument is a month in the past of the
@@ -183,16 +184,23 @@
 	   (and (= (car m) (car n))
 		(< (cadr m) (cadr n))))))
 
-(define (month-diff m n)
-  ; Exclusive
-  1)
+;; Returns the number of months between from f and to t. The first
+;; month is included in the count, the last month is not.
+(define (month-diff f t)
+  (if (month-valid? f)
+      (if (month-valid? t)
+	  (let ((F (+ (* (car f) 12) (cadr f)))
+		(T (+ (* (car t) 12) (cadr t))))
+	    (- T F))
+	  (error 'month-diff "Second argument is not a valid month" t))
+      (error 'month-diff "First argument is not a valid month" f)))
 
 (define (month-tests!)
   (display "[test] month ")
   (unit-test 'month-valid? (month-valid? '(2023 5)))
   (unit-test 'month-valid?-bad-year (not (month-valid? '(999 8))))
   (unit-test 'month-valid?-bad-month (not (month-valid? '(2023 -5))))
-  (unit-test 'string->month (equal? (string->month "2023-01" '(2023 1))))
+  (unit-test 'string->month (equal? (string->month "2023-01") '(2023 1)))
   (unit-test 'string->month-bad-month (not (string->month "2023-13")))
   (unit-test 'string->month-nonumber-year (not (string->month "YYYY-01")))
   (unit-test 'string->month-nonumber-month (not (string->month "2023-MMM")))
@@ -201,11 +209,13 @@
   (unit-test 'month->string-bad-month (with-handler (lambda (x) #t) (month->string '(2023 13)) #f))
   (unit-test 'month<? (month<? '(2023 5) '(2023 6)))
   (unit-test 'month<?-cross-year (month<? '(2022 12) '(2023 1)))
-  (unit-test 'month<?-equal (not (month<? '(2023 1) '(2023 1))))
-  (unit-test 'month<?-greater (not (month<? '(2023 1) '(2023 2))))
-  (unit-test 'month-equal? (month-equal? '(2023 4) '(2023 4)))
-  (unit-test 'month-equal? (not (month-equal? '(2023 4) '(2023 5))))
-  ;; Difference - exclusive end
+  (unit-test 'month<?-is-equal (not (month<? '(2023 1) '(2023 1))))
+  (unit-test 'month<?-greater (not (month<? '(2023 1) '(2023 1))))
+  (unit-test 'month-equal? (month=? '(2023 4) '(2023 4)))
+  (unit-test 'month-equal?-not (not (month=? '(2023 4) '(2023 5))))
+  (unit-test 'month-diff-1 (= (month-diff '(2023 1) '(2023 2)) 1))
+  (unit-test 'month-diff-11 (eq? (month-diff '(2023 1) '(2023 12)) 11))
+  (unit-test 'month-diff (= (month-diff '(2023 1) '(2022 2)) -11))
   (print " ok."))
 
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