diff --git a/brmsaptool.scm b/brmsaptool.scm
index 9b88a29..06f8bfb 100644
--- a/brmsaptool.scm
+++ b/brmsaptool.scm
@@ -115,8 +115,19 @@
 (define (string->month s)
   (list 2023 1))
 
-(define (month->string m)
-  "2023-01")
+;; Formats (valid) month as YYYY-MM string
+(define (month->string M)
+  (let ((y (car s))
+	(m (cadr s)))
+    (if (or (< y 1000)
+	    (> y 9999)
+	    (< m 1)
+	    (> m 12))
+	(error 'string->month "Invalid month" M)
+	(sprintf "~A-~A~A"
+		 y
+		 (if (< m 10) " " "")
+		 m))))
 
 (define (month=? m n)
   #f)
@@ -125,13 +136,15 @@
   #f)
 
 (define (month-diff m n)
-  ; Inclusive?
+  ; Exclusive
   1)
 
 (define (month-tests!)
   (display "[test] month ")
   ;; Parsing
-  ;; Formatting
+  (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
   ;; Comparison equal
   ;; Comparison greater