You can do that

iex(1)> d = DateTime.utc_now
%DateTime{calendar: Calendar.ISO, day: 18, hour: 6, microsecond: {876000, 6},
 minute: 8, month: 9, second: 59, std_offset: 0, time_zone: "Etc/UTC",
 utc_offset: 0, year: 2016, zone_abbr: "UTC"}
iex(2)> DateTime.to_string(d)
"2016-09-18 06:08:59.876000Z" 
iex(3)> s = "#{d.year}/#{d.month}/#{d.day}"
"2016/9/18"

There‘s also a standalone strftime implemention:
https://github.com/plataformatec/nimble_strftime

Most things calendar did found it‘s way into elixir core in the last few versions, so I tend to suggest it (as well as timex) less and less.

Edited to clarify:

Another way is - using this Calendar library:

iex(1)> nbo = Calendar.DateTime.now! "Africa/Nairobi"
  %DateTime{calendar: Calendar.ISO, day: 23, hour: 12, microsecond: {149378, 6},
    minute: 31, month: 7, second: 54, std_offset: 0, time_zone: "Africa/Nairobi",
    utc_offset: 10800, year: 2016, zone_abbr: "EAT"}
iex(2)> nbo |> Calendar.DateTime.shift_zone!("Europe/Vienna")
            |> Calendar.DateTime.Format.iso8601 
  "2016-07-23T11:31:54.149378+02:00"

First we create a timezone for Nairobi. The library uses the names in this list, so we define Africa/Nairobi as timezone name.
On the line iex(2) is the command to get the current date and time formatted as universal date time. We convert this into the Europe/Vienna timezoneand then we format the date and time to ISO 8601. There are more than 20 formats to choose from and if this is not covering your case it is possible to define custom formats.

The library must be loaded, for example by adding a dependency to mix.exs ({:calendar, “~> 0.16.0”}) and then run iex -S mix.

From my book Erlang and Elixir for Imperative Programmers, page 120.