Ensure duration parsing is consistent across DST changes

Previously `ActiveSupport::Duration.parse` used `Time.current` and
`Time#advance` to calculate the number of seconds in the duration
from an arbitrary collection of parts. However as `advance` tries to
be consistent across DST boundaries this meant that either the
duration was shorter or longer depending on the time of year.

This was fixed by using an absolute reference point in UTC which
isn't subject to DST transitions. An arbitrary date of Jan 1st, 2000
was chosen for no other reason that it seemed appropriate.

Additionally, duration parsing should now be marginally faster as we
are no longer creating instances of `ActiveSupport::TimeWithZone`
every time we parse a duration string.

Fixes #26941.

1 files changed, 3 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index 862dabb1e8..82322291d0 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -7,6 +7,8 @@ module ActiveSupport
   #
   #   1.month.ago       # equivalent to Time.now.advance(months: -1)
   class Duration
+    EPOCH = ::Time.utc(2000)
+
     attr_accessor :value, :parts
 
     autoload :ISO8601Parser,     "active_support/duration/iso8601_parser"
@@ -140,8 +142,7 @@ module ActiveSupport
     # If invalid string is provided, it will raise +ActiveSupport::Duration::ISO8601Parser::ParsingError+.
     def self.parse(iso8601duration)
       parts = ISO8601Parser.new(iso8601duration).parse!
-      time  = ::Time.current
-      new(time.advance(parts) - time, parts)
+      new(EPOCH.advance(parts) - EPOCH, parts)
     end
 
     # Build ISO 8601 Duration string for this duration.