[Proposal] Support additional time units

[Parker Selbert]

Problem

Functions that convert or work with time units, i.e. `System.convert_time_unit/1`, `DateTime.add/3` and `Time.add/3` are limited to working with a subset of possible time units. The current subset of named units includes `:second`, `:millisecond`, `:microsecond`, and `:nanosecond`. It is extremely common to manipulate dates and times by larger units such as minutes, hours or days. Currently we have to convert any values to seconds and then use that to change the value, which isn't particularly explicit or expressive. For example, to get the date for one week from now I need to write:

DateTime.add(DateTime.utc_now(), 7 * 24 * 60 * 60, :second)

Proposal

I propose the addition of larger pre-defined units to make conversion more expressive. Units could include some or all of the following:

• `:minute`
• `:hour`
• `:day`
• `:week`

Some larger units such as `month` or `year` are not fixed or standard units and could be omitted.

This enables more expressive time calculations such as "one hour from now", "one day from now", etc.:

utc_now = DateTime.utc_now()
DateTime.add(utc_now, 30, :minute)
DateTime.add(utc_now, 1, :hour)

DateTime.add(utc_now, 1, :day)

In my experience it is extremely common to find module attributes or variables that declar a unit of time to make larger values comprehensible. It is much clearer to specify `1, :day` instead of `@one_day_in_seconds, :second` or something similar.

[José Valim]

Thank you for the proposal. I believe this would be a welcome change. My only question is if we should support them in the Calendar types or directly on the System module. Given the System module does many conversions between time units, supporting those there is slightly more complicated.

Perhaps we start with support on Calendar types and then we move it to System if necessary? This means:

1. Support minute and hour in Time

2. Support week in Date

3. Support minute, hour, day and week in NaiveDateTime

4. Support minute, hour, day and week in DateTime

--
You received this message because you are subscribed to the Google Groups "elixir-lang-core" group.
To unsubscribe from this group and stop receiving emails from it, send an email to elixir-lang-co...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/7ca22377-aadc-4e5b-a57f-ad0c52fc5ba8%40googlegroups.com.

[Wojtek Mach]



Just a word of warning, I believe that while supporting minutes, hours, and days is rather trivial since we have such fields on the structs and so we could just increment them, supporting weeks is trickier - are all weeks composed of 7 days or there are calendars where this is not true? Maybe se should start without weeks then? Or there will be a callback for them? Leap seconds may or may not be a concern too.

On 3 Jun 2020, at 08:45, José Valim <jose....@dashbit.co> wrote:



To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4KTjtL-4K3-hj%2Bf%3D7sqHrkOJ52_C9VJ9u_Ekc35C_dY_A%40mail.gmail.com.

[José Valim]

Good point Wojtek.

Unfortunately this concern also applies to hours and seconds, which can change between calendars. I believe seconds today already operates as ISO seconds, so I would just document that all units are ISO units.

To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/FF8AE738-AF6D-4B5C-8EB9-55F86DC49D8D%40wojtekmach.pl.

[Ulisses De Almeida]

Well, if you implementing a fantasy world game, a week can have different days.

For example, in forgotten realms, a week has 10 days.

https://forgottenrealms.fandom.com/wiki/Calendar_of_Harptos

To view this discussion on the web visit https://groups.google.com/d/msgid/elixir-lang-core/CAGnRm4Je5HVRyv8CCKjoTSOVsh7C9eVB7ZQa9WbNN2N7KaDBgQ%40mail.gmail.com.

--

Ulisses H. F. de Almeida

ulissesa...@gmail.com
ani...@gmail.com

[Blake Kostner]

Looking at the Elixir code for this, it's mostly routed to `System.convert_time_unit/3` which is a wrapper to `erlang.convert_time_unit/3`. Would it make more sense to try upstreaming this into erlang/otp?

[luk...@niemier.pl]

It will not work, because second is the last time unit that has constant length. Minute has length in range 59..61 seconds, longer units will have broader ranges. So it can be only handled in connection with calendar.