docs: add changelog and upgrade guide

Author: kenjis

user_guide_src/source/changelogs/v4.3.0.rst

@@ -42,6 +42,22 @@ For example, the Exit code has been changed like the following:
 - If an uncaught ``CastException`` occurs, the Exit code is ``EXIT_CONFIG`` (= ``3``) instead of ``9``.
 - If an uncaught ``DatabaseException`` occurs, the Exit code is ``EXIT_DATABASE`` (= ``8``) instead of ``17``.
 
+.. _v430-time-fix:
+
+Time
+----
+
+The following methods of the :doc:`Time <../libraries/time>` class had bugs that changed the state of the current object. To fix these bugs, the Time class has been fixed:
+    - ``add()``
+    - ``modify()``
+    - ``setDate()``
+    - ``setISODate()``
+    - ``setTime()``
+    - ``sub()``
+
+- Now the ``Time`` class extends ``DateTimeImmutable`` and is completely immutable.
+- ``TimeLegacy`` class has been added for backward compatibility, which behaves the same as the unmodified ``Time`` class.
+
 Others
 ------
 

user_guide_src/source/installation/upgrade_430.rst

@@ -80,6 +80,15 @@ Validation Changes
 - ``ValidationInterface`` has been changed. Implemented classes should likewise add the methods and the parameters so as not to break LSP. See :ref:`v430-validation-changes` for details.
 - The return value of  ``Validation::loadRuleGroup()`` has been changed ``null`` to ``[]`` when the ``$group`` is empty. Update the code if you depend on the behavior.
 
+Time Fixes
+==========
+
+Due to bug fixes, some methods in :doc:`Time <../libraries/time>` has been changed the behavior from mutable to immutable. See :ref:`ChangeLog <v430-time-fix>` for details.
+
+If you need the behavior of ``Time`` before the modification, a compatible ``TimeLegacy`` class has been added. Please replace all ``Time`` with ``TimeLegacy`` in your application code.
+
+But ``TimeLegacy`` is deprecated. So we recommend you update your code.
+
 .. _upgrade-430-stream-filter:
 
 Capturing STDERR and STDOUT streams in tests

user_guide_src/source/libraries/time.rst

@@ -6,8 +6,12 @@ CodeIgniter provides a fully-localized, immutable, date/time class that is built
 extension's features to convert times across timezones and display the output correctly for different locales. This class
 is the ``Time`` class and lives in the ``CodeIgniter\I18n`` namespace.
 
-.. note:: Since the Time class extends DateTimeImmutable, if there are features that you need that this class doesn't provide,
-    you can likely find them within the `DateTime <https://www.php.net/manual/en/class.datetimeimmutable.php>`_  class itself.
+.. note:: Since the Time class extends ``DateTimeImmutable``, if there are features that you need that this class doesn't provide,
+    you can likely find them within the `DateTimeImmutable <https://www.php.net/manual/en/class.datetimeimmutable.php>`_  class itself.
+
+.. note:: Prior to v4.3.0, the Time class extended ``DateTime`` and some inherited methods changed
+    the current object state. The bug was fixed in v4.3.0. If you need the old Time class for backward
+    compatibility, you can use deprecated ``TimeLegay`` class for the time being.
 
 .. contents::
     :local: