Clojure java-time: getting instant with millis

Question

I'm trying to convert a local-date into an instant with millis using java-time, but instant returns a timestamp without millis.

(def UTC (java-time/zone-id "UTC")

(defn local-date-to-instant [local-date] 
  (-> local-date
      (.atStartOfDay UTC)
      java-time/instant))

(local-date-to-instant (java-time/local-date))
=> #object[java.time.Instant 0xdb3a8c7 "2021-05-13T00:00:00Z"]

but

(java-time/instant)
=> #object[java.time.Instant 0x1d1c27c8 "2021-05-13T13:12:31.782Z"]

The service downstream expects a string in this format: yyyy-MM-ddTHH:mm:ss.SSSZ.

Answer 1

Create a DateTimeFormatter that prints an ISO instant with milliseconds (3 fractional digits) even if they are zeroes:

(ns steffan.overflow
  (:require [java-time :as jt])
  (:import (java.time.format DateTimeFormatterBuilder)))

(def iso-instant-ms-formatter
  (-> (DateTimeFormatterBuilder.) (.appendInstant 3) .toFormatter))

Example of use:

(def today-inst (jt/truncate-to (jt/instant) :days))

(str today-inst)                                ; => "2021-05-13T00:00:00Z"
(jt/format iso-instant-ms-formatter today-inst) ; => "2021-05-13T00:00:00.000Z"