diff --git a/brmsaptool.scm b/brmsaptool.scm
index 4675adb..8b4c553 100644
--- a/brmsaptool.scm
+++ b/brmsaptool.scm
@@ -147,8 +147,14 @@
        (valid-month? n)
        (equal? m n)))
 
+;; Returns true if the first argument is a month in the past of the
+;; second argument month
 (define (month<? m n)
-  #f)
+  (and (month-valid? m)
+       (month-valid? n)
+       (or (< (car m) (car n))
+	   (and (= (car m) (car n))
+		(< (cadr m) (cadr n))))))
 
 (define (month-diff m n)
   ; Exclusive
@@ -163,11 +169,13 @@
   (unit-test 'month->string (equal? (month->string '(2023 1)) "2023-01"))
   (unit-test 'month->string-bad-year (with-handler (lambda (x) #t) (month->string '(999 12)) #f))
   (unit-test 'month->string-bad-month (with-handler (lambda (x) #t) (month->string '(2023 13)) #f))
-  ;; Comparison less
+  (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))))
-  ;; Comparison greater
-  ;; Difference - TODO: inclusive both ends?
+  ;; Difference - exclusive end
   (print " ok."))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;