# DateTime

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`](https://hexdocs.pm/elixir/Calendar.html#get_time_zone_database/0), which defaults to [`Calendar.UTCOnlyTimeZoneDatabase`](https://hexdocs.pm/elixir/Calendar.UTCOnlyTimeZoneDatabase.html) which **only handles "Etc/UTC"** datetimes and returns `{:error, :utc_only_time_zone_database}` for any other time zone.

* need lib to handle timezones, **use timex package**

```elixir
dt = DateTime.utc_now()
dt = ~U[2020-05-29 02:15:00Z] #Only UTC Timezone

# Change to first of month
%{dt | day: 1, hour: 0, minute: 0, second: 0, microsecond: {0, 0}}

dt |> DateTime.add(60, :second) #Expected :second, :millisecond, :microsecond, :nanosecond
```

### Native DateTime

**No timezone data** so because of daylight savings could occur twice or never

```elixir
naive = ~N[2000-01-01 23:00:07]
naive.year #2000
naive.second #7
dt.year #2020

NaiveDateTime.add(~N[2014-10-02 00:29:10.021], 21, :second) 
```

### UNIX

```elixir
nowUTC = DateTime.utc_now() |> DateTime.to_unix(:millisecond)

DateTime.from_unix(1594370959) #{:ok, ~U[2020-07-10 08:49:19Z]}
DateTime.from_unix!(1594370959)
```

#### Comparison

Normally < and > don't work well

```elixir
DateTime.compare(now, se.start_time) #:lt, :eq, :gt
DateTime.compare(~U[2021-08-21 04:00:00Z], ~U[2021-09-21 04:00:00Z]) #:lt
```

```elixir
case DateTime.compare(end_datetime, now) do
	:lt ->
    get_current_month_period(now, Timex.shift(start_datetime, months: 1))

	:eq ->
		%{period_start: start_datetime, period_end: end_datetime}

	:gt ->
		%{period_start: start_datetime, period_end: end_datetime}
end
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://openai.gitbook.io/code-cheatsheets/all/elixir/datetime.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.