DateTime.new with timezone (Rails)

Question

DateTime.new takes a timezone parameter as the 7th argument as an integer offset.

DateTime.new(2001,2,3,4,5,6,'-7')

However, since I don't easily know whether a given time falls in Daylight or Standard, I would rather do something like:

DateTime.new(2001,2,3,4,5,6,'Eastern Time (US & Canada)')

Please advise

Answer 1

If you're using Rails your best bet is:

cur_zone = Time.zone
begin
  Time.zone = 'Eastern Time (US & Canada)'
  datetime = Time.zone.local(2001, 2, 3, 4, 5, 6)
ensure
  Time.zone = cur_zone
end

Or if you've already set Time.zone when you authenticate your uses, or in your application.rb config file then you can just use the Time.zone.local line by itself.

Answer 2

Idea found in: https://stackoverflow.com/a/41584881/3837660

In e.g. Rails 5.2.3

DateTime.included_modules.third
# => DateAndTime::Zones

which has #in_time_zone. (https://api.rubyonrails.org/classes/DateAndTime/Zones.html)

Allowing you to do the following:

#########################################
# Summer example
#
DateTime.new.in_time_zone(
  'Eastern Time (US & Canada)'
).change(
  year: 2017,
  month: 7,
  day: 31,
  hour: 7,
  min: 59,
  sec: 23
)
#
# => Mon, 31 Jul 2017 07:59:23 EDT -04:00
#
#########################################

#########################################
# Winter example
DateTime.new.in_time_zone(
  'Eastern Time (US & Canada)'
).change(
  year: 2017,
  month: 12,
  day: 31,
  hour: 7,
  min: 59,
  sec: 23
)
#
# => Sun, 31 Dec 2017 07:59:23 EST -05:00
#
#########################################

Note that DateTime#change is also an addition from Rails. See, e.g., https://api.rubyonrails.org/v5.2.3/classes/DateTime.html#method-i-change for details.