DateTime Format to string?

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"

thank you!

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.

But that won’t work out of the box, because Calendar is behaviour http://elixir-lang.org/docs/stable/elixir/Calendar.html and without defining callbacks by you or any other model it won’t work.

iex(4)> d = Calendar.DateTime.now! "Africa/Nairobi"
** (UndefinedFunctionError) function Calendar.DateTime.now!/1 is undefined (module Calendar.DateTime is not available)
    Calendar.DateTime.now!("Africa/Nairobi")

Correct.

Calendar must have been loaded before.

Question, by loading it before I assume you mean using the hex module Calendar right?

I just realized @kujua were talking about a third party library GitHub - lau/calendar: date-time and time zone handling in Elixir · GitHub, and not about the Elixir’s standard library Calendar http://elixir-lang.org/docs/stable/elixir/Calendar.html. I think he should mention about what library he was writing in his response, to avoid confusion.

Sorry for being not clear - was a long evening yesterday

The mix file has a dependency {:calendar, “~> 0.16.0”}
Starting in the directory where the file resides: iex -S mix. This will load all dependencies.

As @sztosz rightly says this is the library https://github.com/lau/calendar which has more features than the built-in one in 1.3. Also, version 0.16.0 only works with Elixir 1.3, because it accepts the built-in calendar 1.3 types.

Erlang’s calendar is also good, I use it to get string dates with flexible format like that:

defmodule Util.Time do
  @moduledoc """
  Time helper functions.
  """

  @doc """
  Get current time as string using Erlangs calendar module.
  """
  @spec now_to_string() :: String
  def now_to_string() do
    {{year, month, day}, {hour, minute, second}} = :calendar.local_time()
    "#{day}.#{month |> zero_pad}.#{year} #{hour |> zero_pad}:#{minute |> zero_pad}:#{second |> zero_pad}"
  end

  @spec zero_pad(Integer, Integer) :: String
  defp zero_pad(number, amount \\ 2) do
     number
     |> Integer.to_string
     |> String.rjust(amount, ?0)
  end

end

@kujua I went with the Calendar module. The inbuilt one seemed pretty simple and didn’t do diffs, etc. Thanks for everyone’s help!