Fix Time#advance to work with dates before 1001-03-07

In #10634 the behavior of Time#advance was changed to maintain a
proleptic gregorian calendar for dates before calendar reform. However
it didn't full address dates a long time before calendar reform and
they gradually drift away from the proleptic calendar the further you
go back in time. Fix this by always converting the date to gregorian
before calling advance which sets the reform date to -infinity.

3 files changed, 17 insertions, 2 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index b2330f2c9d..63e2e44597 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,17 @@
+*   Fix `Time#advance` to work with dates before 1001-03-07
+
+    Before:
+    
+        Time.utc(1001, 3, 6).advance(years: -1) # => 1000-03-05 00:00:00 UTC
+    
+    After
+    
+        Time.utc(1001, 3, 6).advance(years: -1) # => 1000-03-06 00:00:00 UTC
+
+    Note that this doesn't affect `DateTime#advance` as that doesn't use a proleptic calendar.
+
+    *Andrew White*
+
 *   In Zeitwerk mode, engines are now managed by the `main` autoloader. Engines may reference application constants, if the application is reloaded and we do not reload engines, they won't use the reloaded application code.
 
     *Xavier Noria*
diff --git a/activesupport/lib/active_support/core_ext/time/calculations.rb b/activesupport/lib/active_support/core_ext/time/calculations.rb
index 120768dec5..f09a6271ad 100644
--- a/activesupport/lib/active_support/core_ext/time/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/time/calculations.rb
@@ -170,8 +170,7 @@ class Time
       options[:hours] = options.fetch(:hours, 0) + 24 * partial_days
     end
 
-    d = to_date.advance(options)
-    d = d.gregorian if d.julian?
+    d = to_date.gregorian.advance(options)
     time_advanced_by_date = change(year: d.year, month: d.month, day: d.day)
     seconds_to_advance = \
       options.fetch(:seconds, 0) +
diff --git a/activesupport/test/core_ext/time_ext_test.rb b/activesupport/test/core_ext/time_ext_test.rb
index 7078f3506d..590b81b770 100644
--- a/activesupport/test/core_ext/time_ext_test.rb
+++ b/activesupport/test/core_ext/time_ext_test.rb
@@ -514,6 +514,8 @@ class TimeExtCalculationsTest < ActiveSupport::TestCase
     assert_equal Time.local(1582, 10, 15, 15, 15, 10), Time.local(1582, 10, 14, 15, 15, 10).advance(days: 1)
     assert_equal Time.local(1582, 10, 5, 15, 15, 10), Time.local(1582, 10, 4, 15, 15, 10).advance(days: 1)
     assert_equal Time.local(1582, 10, 4, 15, 15, 10), Time.local(1582, 10, 5, 15, 15, 10).advance(days: -1)
+    assert_equal Time.local(999, 10, 4, 15, 15, 10), Time.local(1000, 10, 4, 15, 15, 10).advance(years: -1)
+    assert_equal Time.local(1000, 10, 4, 15, 15, 10), Time.local(999, 10, 4, 15, 15, 10).advance(years: 1)
   end
 
   def test_last_week