Date and Time

Working with time in Elixir.

Table of Contents

Time

Elixir has some modules which work with time. Let’s start with getting the current time:

iex> Time.utc_now
~T[19:39:31.056226]

Note that we have a sigil which can be used to create a Time struct as well:

iex> ~T[19:39:31.056226]
~T[19:39:31.056226]

You can learn more about sigils in the lesson about sigils. It is easy to access parts of this struct:

iex> t = ~T[19:39:31.056226]
~T[19:39:31.056226]
iex> t.hour
19
iex> t.minute
39
iex> t.day
** (KeyError) key :day not found in: ~T[19:39:31.056226]

But there is a catch: as you may have noticed, this struct only contains the time within a day, no day/month/year data is present.

Date

Contrary to Time, a Date struct has info about the current date, without any info about current time.

iex> Date.utc_today
~D[2028-10-21]

And it has some useful functions to work with dates:

iex> {:ok, date} = Date.new(2020, 12,12)
{:ok, ~D[2020-12-12]}
iex> Date.day_of_week date
6
iex> Date.leap_year? date
true

day_of_week/1 calculates which day of the week a given date is on. In this case it’s Saturday. leap_year?/1 checks whether this is a leap year. Other functions can be found in doc.

NaiveDateTime

There are two kinds of structs which contain both date and time at once in Elixir. The first is NaiveDateTime. Its disadvantage is lack of timezone support:

iex(15)> NaiveDateTime.utc_now
~N[2029-01-21 19:55:10.008965]

But it has both the current time and date, so you can play with adding time, for example:

iex> NaiveDateTime.add(~N[2018-10-01 00:00:14], 30)
~N[2018-10-01 00:00:44]

DateTime

The second, as you may have guessed from the title of this section, is DateTime. It does not have the limitations noted in NaiveDateTime: it has both time and date, and supports timezones. But be aware about timezones. The official docs state:

Many functions in this module require a time zone database. By default, it uses the default time zone database returned by Calendar.get_time_zone_database/0, which defaults to Calendar.UTCOnlyTimeZoneDatabase which only handles “Etc/UTC” datetimes and returns {:error, :utc_only_time_zone_database} for any other time zone.

Also, note that you can create a DateTime instance from the NaiveDateTime, just by providing the timezone:

iex> DateTime.from_naive(~N[2016-05-24 13:26:08.003], "Etc/UTC")
{:ok, #DateTime<2016-05-24 13:26:08.003Z>}

Working with timezones

As we have noted in the previous chapter, by default Elixir does not have any timezone data. To solve this issue, we need to install and set up the tzdata package. After installing it, you should globally configure Elixir to use Tzdata as timezone database:

config :elixir, :time_zone_database, Tzdata.TimeZoneDatabase

Let’s now try creating time in Paris timezone and convert it to New York time:

iex> paris_datetime = DateTime.from_naive!(~N[2019-01-01 12:00:00], "Europe/Paris")
#DateTime<2019-01-01 12:00:00+01:00 CET Europe/Paris>
iex> {:ok, ny_datetime} = DateTime.shift_zone(paris_datetime, "America/New_York")
{:ok, #DateTime<2019-01-01 06:00:00-05:00 EST America/New_York>}
iex> ny_datetime
#DateTime<2019-01-01 06:00:00-05:00 EST America/New_York>

As you can see, time changed from 12:00 Paris time to 6:00, which is correct - time difference between the two cities is 6 hours.

This is it! If you want to work with other advanced functions you may want to consider looking futher into docs for Time, Date, DateTime and NaiveDateTime You should also consider Timex and Calendar which are powerful libraries to work with time in Elixir.

Caught a mistake or want to contribute to the lesson? Edit this page on GitHub!