How do you format date-times in RFC 3339 style?

[tantalum <sph@posteo.eu>]

i once wrote a rfc3339 reader/writer and worked through some gotchas, maybe the code contains hints that are helpful.
for example, here is a function that converts from a timestamp to a rfc3339 date string:

   (define* (utc->rfc3339 a #:optional (offset 0) (seconds-fraction 0))
     "integer:posix-time -> string"
     (let
       ( (date-time
           (let (t (gmtime (+ a offset)))
             (apply
               (l (y m d h mi s)
                 (string-append y "-"
                   m "-"
                   d "T"
                   h ":"
                   mi ":"
                   s
                   (if (zero? seconds-fraction) ""
                     (string-append "." (number->string seconds-fraction)))))
               (map number->padded-string
                 (list (+ 1900 (tm:year t)) (+ 1 (tm:mon t))
                   (tm:mday t) (tm:hour t) (tm:min t) (tm:sec t))))))
         (offset
           (if (zero? offset) "Z"
             (apply
               (l (sign numbers)
                 (string-append sign (string-join (map number->padded-string numbers) ":")))
               (let* ((hms (drop-right (utc-duration->hms offset) 1)) (hours (first hms)))
                 (if (any negative? hms) (list "-" (map (l (a) (* -1 a)) hms)) (list "+" hms)))))))
       (string-append date-time offset)))

l: lambda, first: car

the code passes these tests (examples of valid strings):
https://github.com/sph-mn/sph-lib/blob/master/modules/test/module/sph/time/rfc3339.scm

for reference, the complete code: https://github.com/sph-mn/sph-lib/blob/master/modules/sph/time/rfc3339.scm