How to compare time in elixir

I want to make a schedule a job for sending emails or texting messages.
And I found rihanna.

But before do that, I need a more understanding of DateTime…
I get an datetime input from customer in this format
date = “2019-09-25”
time = “16:00”

  1. How can I format this input to DateTime format? I tried.
  def build_datetime(date, time) do
    with {:ok, datetime, 0} <- DateTime.from_iso8601("#{date} #{time}:00"),
      do: datetime

But got an error

{:error, :missing_offset}

  1. I want to check user’s datetime is in future( 30 mins future from now on). So I would do this…
def schedule_datetime_is_future?(schedule_datetime) do
  DateTime.diff(schedule_datetime, DateTime.utc_now) > 30 * 60
  1. in rihanna docs says
  {IO, :puts, ["Hello"]},
  at: DateTime.from_naive!(~N[2018-07-01 12:00:00], "Etc/UTC")

So before I have to compare datetime, should I convert datetime to UTC datetime?
It works in different timezone?

1 Like

You need to specify the timezone or add time offset from UTC. So you can do one of the following:

iex(1)> date = "2019-09-25"                         

iex(2)> time = "16:00"                              

iex(3)> DateTime.from_iso8601("#{date} #{time}:00Z")
{:ok, #DateTime<2019-09-25 16:00:00Z>, 0}

iex(4)> DateTime.from_iso8601("#{date} #{time}:00+00:00")
{:ok, #DateTime<2019-09-25 16:00:00Z>, 0}

In the first example, you specify the timezone is UTC via Z and in the second example, you specify the offset from the UTC timezone. More info here.

I still don’t understand. :frowning:
Users select a date time in front end (it is their local time zone). Let’s say, they pick 2018-09-29 and time 16:00
How can I convert this time to their time zone? So I can schedule job at their time zone time.

I work on a project that heavily uses time/scheduling. I highly suggest only store UTC time in your application database, and convert it client side.


You need to know what the utc_offset is for their given timezone. For instance, if a user is from San Francisco, they would be in the Pacific Time Zone, which has an offset of -08:00 from UTC. Then, the correct DateTime would be:

{_, time_in_utc, offset} = DateTime.from_iso8601("#{date} #{time}:00-08:00").

You can then store all input times in UTC, schedule using UTC time and convert it to the corresponding time in their time zone using offset (for instance, to display in the UI.)

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That is not sufficient. You need to know their time zone, because UTC offset at this moment in time does not tell you what their UTC offset would be at some other moment in time.


That is a problem what I want to solve.

  1. user picks datetime from datetime picker ui.
  2. my app accepts it as string date - “2018-09-26”, time - “16:00”
  3. I need to find out users’ timezone info.
  4. convert it to utc time format with timezone info
  5. check if users’ datetime is in the future in utc time format
    6.if users’ datetime is in the future, make a schedule job.

This is a classic example of the shittiness of web browsers and Javascript library design. There has been no API for this. There are ways to get at it in some browsers, using the internationalization API. Your best bet is probably to use the moment.js timezone functions. Alternatively you can do what a lot of web-based apps do: ask the user to select their time zone in preferences.