Rails 8 – New ActiveSupport Timezone Defaults

Rails 8 introduces a new default value for

ActiveSupport.to_time_preserves_timezone

# config/initializers/new_framework_defaults_8_0.rb
#
# Specifies whether `to_time` methods preserve the UTC offset of their receivers or preserve the timezone.
# If set to `:zone`, `to_time` methods will use the timezone of their receivers.
# If set to `:offset`, `to_time` methods will use the UTC offset.
# If `false`, `to_time` methods will convert to the local system UTC offset instead.
#
# Rails.application.config.active_support.to_time_preserves_timezone = :zone

It was not clear to me what this changes for my application or where problems might hide. So I figured it out.

require 'active_support/all'
Time.zone = 'America/New_York'

In the United States, daylight saving time begins on the second Sunday in March and ends on the first Sunday in November, with the time change occurring at 2:00 a.m. local time. I chose the borderline case - November 1st.

ActiveSupport.to_time_preserves_timezone = :offset

t = Time.zone.parse('1/Nov/2025 12:30:00').to_time
=> 2025-11-01 12:30:00 -0400

t.zone
=> nil

t + 1.day
=> 2025-11-02 12:30:00 -0400

Notice the lack of time zone information in the result object. The transition from daylight saving time was not completed.

ActiveSupport.to_time_preserves_timezone = :zone

t = Time.zone.parse('1/Nov/2025 12:30:00').to_time
=> 2025-11-01 12:30:00 -0400

t.zone
=> #<ActiveSupport::TimeZone:0x000000011c9aff20 @name="America/New_York", @tzinfo=#<TZInfo::DataTimezone: America/New_York>, @utc_offset=nil>

t + 1.day
=> 2025-11-02 12:30:00 -0500

But with the new Rails default (:zone), the to_time method returns an object with timezone information, which helps to correctly make calculations taking the daylight saving time transition into account.