I'm having fun with ncm's programming challenge. Here's a Scheme test harness for it, assuming you call the test function timestamp-increment!

(define test-values
  '(("19991231235959" . "20000101000000")
    ("20000228235959" . "20000229000000")
    ("20000229235959" . "20000301000000")
    ("20040228235959" . "20040229000000")
    ("21000228235959" . "21000301000000")
    ("19000228235959" . "19000301000000")
    ("16000228235959" . "16000229000000")))

 (define (test-answer q-a-pair)
  (let ((a (string-copy (car q-a-pair))))
    (timestamp-increment! a)
    (if (string=? a (cdr q-a-pair))
	'ok
	(cons a q-a-pair))))

 (define test-results
  (map test-answer test-values))