Saving datetime in UTC isn't accurate sometimes

Question

In general, best practice when dealing with dates is to store them in UTC and convert back to whatever the user expects within the application layer.

That doesn\'t neces

Answer 1

I propose that you still use the first option but with a little hack: in essence, you can switch off the time zone conversion for the desired attribute and use a custom setter to overcome the conversion during attribute writes.

The trick saves the time as a fake UTC time. Although technically it has an UTC zone (as all the times are saved in db in UTC) but by definition it shall be interpreted as local time, regardless of the current time zone.

class Model < ActiveRecord::Base
  self.skip_time_zone_conversion_for_attributes = [:start_time]

  def start_time=(time)
    write_attribute(:start_time, time ? time + time.utc_offset : nil)
  end
end

Let's test this in rails console:

$ rails c
>> future_time = Time.local(2020,03,30,11,55,00)
=> 2020-03-30 11:55:00 +0200

>> Model.create(start_time: future_time)
D, [2016-03-15T00:01:09.112887 #28379] DEBUG -- :    (0.1ms)  BEGIN
D, [2016-03-15T00:01:09.114785 #28379] DEBUG -- :   SQL (1.4ms)  INSERT INTO `models` (`start_time`) VALUES ('2020-03-30 11:55:00')
D, [2016-03-15T00:01:09.117749 #28379] DEBUG -- :    (2.7ms)  COMMIT
=> #

Note that Rails saved the time as a 11:55, in a "fake" UTC zone.

Also note that the time in the object returned from create is wrong because the zone is converted from the "UTC" in this case. You would have to count with that and reload the object every time after setting the start_time attribute, so that the zone conversion skipping can take place:

>> m = Model.create(start_time: future_time).reload
D, [2016-03-15T00:08:54.129926 #28589] DEBUG -- :    (0.2ms)  BEGIN
D, [2016-03-15T00:08:54.131189 #28589] DEBUG -- :   SQL (0.7ms)  INSERT INTO `models` (`start_time`) VALUES ('2020-03-30 11:55:00')
D, [2016-03-15T00:08:54.134002 #28589] DEBUG -- :    (2.5ms)  COMMIT
D, [2016-03-15T00:08:54.141720 #28589] DEBUG -- :   Model Load (0.3ms)  SELECT  `models`.* FROM `models` WHERE `models`.`id` = 10 LIMIT 1
=> #

>> m.start_time
=> 2020-03-30 11:55:00 UTC

After loading the object, the start_time attribute is correct and can be manually interpreted as local time regardless of the actual time zone.

I really don't get it why Rails behaves the way it does regarding the skip_time_zone_conversion_for_attributes configuration option...

Update: adding a reader

We can also add a reader so that we automatically interpret the saved "fake" UTC time in local time, without shifting the time due to timezone change:

class Model < ActiveRecord::Base
  # interprets time stored in UTC as local time without shifting time
  # due to time zone change
  def start_time
    t = read_attribute(:start_time)
    t ? Time.local(t.year, t.month, t.day, t.hour, t.min, t.sec) : nil
  end
end

Test in rails console:

>> m = Model.create(start_time: future_time).reload
D, [2016-03-15T08:10:54.889871 #28589] DEBUG -- :    (0.1ms)  BEGIN
D, [2016-03-15T08:10:54.890848 #28589] DEBUG -- :   SQL (0.4ms)  INSERT INTO `models` (`start_time`) VALUES ('2020-03-30 11:55:00')
D, [2016-03-15T08:10:54.894413 #28589] DEBUG -- :    (3.1ms)  COMMIT
D, [2016-03-15T08:10:54.895531 #28589] DEBUG -- :   Model Load (0.3ms)  SELECT  `models`.* FROM `models` WHERE `models`.`id` = 12 LIMIT 1
=> #

>> m.start_time
=> 2020-03-30 11:55:00 +0200

I.e. the start_time is correctly interpreted in local time, even though it was stored as the same hour and minute, but in UTC.