Add support for loading a datetime value

[amaraliou]

Currently, there is no way to pass a datetime environment variable. The closest thing we have is the :date type which one can only use for dates. We have recently hit a use case where it would be ideal to support loading :datetime environment variables (we want to set a date and time after which some business logic changes).

This PR adds just that + the validation required to load a datetime value correctly.

[ameykusurkar]

Looking at the documentation:

This method does not function as a validator. If the input string does not match valid formats strictly, you may get a cryptic result. Should consider to use DateTime.strptime instead of this method as possible.

Should we use a standardised format (eg. DateTime.rfc3339(...)) to avoid ambiguity?

[ameykusurkar]

Optionally, we can call the arguument :datetime_rfc3339 if we want.

[benk-gc]

Petition to use ActiveSupport::TimeWithZone. It has proper timezone support, and we can safely (de)serialize values with the iso8601 method which is our general standard elsewhere (e.g. in Pub/Sub messages).

[kevinrobayna]

I came here to talk about the above.

Date.parse or DateTime.parse are Russian roulettes so we def need to use a specific format and agree on it.

[kevinrobayna]

ActiveSupport::TimeWithZone

that would mean adding rails as a dependency, not sure we should do that.

[benk-gc]

@kevinrobayna I think we can just add active_support as a dependency and require the minimal active_support requirements and active_support/time_with_zone like so:

require "active_support"
require "active_support/time_with_zone"

See https://guides.rubyonrails.org/active_support_core_extensions.html#stand-alone-active-support

[kevinrobayna]

that is "rails" 😅

[ameykusurkar]

So I just found this:

DateTime class is considered deprecated. Use Time class.

I'm in two minds about adding a big dependency to a very minimal gem as well. Granted that TimeWithZone comes with a bunch of goodies, but looks like Time does handle the basics of timezones correctly:

irb(main):001> require "time"
=> true
irb(main):002> t = Time.iso8601("2023-10-07T21:15:30+11:00")
=> 2023-10-07 21:15:30 +1100
irb(main):003> t.utc
=> 2023-10-07 10:15:30 UTC

Plus if this gem is used in the context of a Rails app, then we can just do t.in_time_zone to get a full-fledged ActiveSupport::TimeWithZone, so probably not too bad?