DateTime Format to string?

Questions / Help
1

Hi,

I am new to Elixir. I am trying to use the DateTime component to insert a date into MySQL however the there seems to be no way to format a datetime to a string. Can someone point me in the right direction?

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2

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"
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3

thank you!

4

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.

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5

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")
6

Correct.

Calendar must have been loaded before.

7

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

8

I just realized @kujua were talking about a third party library https://github.com/lau/calendar, 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.

9

Sorry for being not clear - was a long evening yesterday :slight_smile:

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.

10

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
11

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

12

How would you format it though?

13

Can you elaborate?

14

How about a format like 20191101T17:01:58+0000 - I derived this in PHP using Ymd\TH:i:sO format.

15

I would use the Calendar library (mentioned earlier in this thread) with the strftime function to customize the DateTime formats however you like.

https://hexdocs.pm/calendar/Calendar.Strftime.html#strftime/3

16

There‘s also a standalone strftime implemention:

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.

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