A trivial Common Lisp library to convert local-time timestamps, existing RFC 3339 strings or Unix epoch times (integer or double float) to a canonical RFC 3339 compliant format in UTC with millisecond granularity. For example

2015-12-28T02:59:00.000Z

This is the same format returned by the JavaScript function

Date.prototype.toISOString()

which was [standardized in ECMA-262 5th edition] (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toISOString).

To be clear, RFC 3339 leave some wriggle room. For example, the literal T above may be a space, something the PostgreSQL project tends to use. But, to paraphrase section 5.1 of the above RFC 3339, by standardizing on a canonical format viz:

• always using timezeone Z (that is, UTC)
• always using millisecond granularity

the resulting time strings are all the same length and all of the same format and so may be sorted and compared as strings and yet provide the expected result as times.

For those people storing times without access to a first class time type (eg. in JSON) such a standard format is desireable.

Of course, using Unix time in milliseconds is easier still (and smaller and faster) but has the drawback that 1451271540000 is rather opaque compared to the above string.

There is but one exported symbol, the generic function rfc3339-string:

CL-USER> (defpackage :example
           (:use :cl :cl-rfc3339 :local-time))
(in-package :example)

(rfc3339-string (now))
"2014-12-18T20:31:35.743Z"

(rfc3339-string (timestamp-to-unix (now)))
"2014-12-18T20:31:50.000Z"

(rfc3339-string *)
"2014-12-18T20:31:50.000Z"

(rfc3339-string "2014-12-18T20:24:46.000001Z")
"2014-12-18T20:24:46.000Z"

(rfc3339-string "2014-12-18T20:24:46.499001Z")
"2014-12-18T20:24:46.499Z"

(rfc3339-string "2014-12-18T20:24:46.4999Z")
"2014-12-18T20:24:46.499Z"

(let ((str "2014-12-18T06:56:45.000Z"))
  (rfc3339-string (+ 173d-3 (timestamp-to-unix (parse-rfc3339-timestring str)))))
"2014-12-18T06:56:45.173Z"

Navigate to your ~/quicklisp/local-projects directory and do git clone https://github.com/gtod/cl-rfc3339.git. Then at your REPL evaluate:

(ql:register-local-projects)
(ql:quickload :cl-rfc3339)

(ql:quickload :cl-rfc3339-test)
(cl-rfc3339-test:run)