An (incomplete) implementation of the strftime format based on rules from http://strftime.org.
import Strftime exposing (format)
format "%d %B %y" Time.utc (Time.millisToPosix 1499000000000) == "02 July 17"
format "%B %d %Y, %-I:%M" Time.utc (Time.millisToPosix 1490000000000) == "March 20 2017, 8:53"